## Overview

related to xmidt-org#25, xmidt-org#78, xmidt-org/scytale#88, xmidt-org/talaria#153 s.t. we want a validation mechanism that is configurable by the application & verifies the spec.

### tl;rd
This pr introduces the initial validation framework, where applications supply validators (satisfying the `Validator interface`) to `NewMsgTypeValidator` and then used to verify the spec.

<details>
<summary> Explanation</summary>

Apps supply validators satisfying:
```go
// Validator is a WRP validator that allows access to the Validate function.
type Validator interface {
	Validate(m Message) error
}
```

and listing which validators are used on known and unknown msg types (where unknown msg types are handled by `defaultValidator`):
```go
var alwaysValidMsg ValidatorFunc = func(msg Message) error { return nil }
msgv, err := NewMsgTypeValidator(
	// Validates known msg types
	m: map[MessageType]Validators{SimpleEventMessageType: {alwaysValidMsg}},
	// Validates unknown msg types
	defaultValidator: alwaysValidMsg)
err = msgv.Validate(Message{Type: SimpleEventMessageType}) // Known msg type
err == nil // True
err = msgv.Validate(Message{Type: CreateMessageType}) // Unknown msg type, uses defaultValidator
err == nil // True
```

if a default validator is not provided, all unknown msg type will **fail** by default
```go
var alwaysValidMsg ValidatorFunc = func(msg Message) error { return nil }
msgv, err := NewMsgTypeValidator( // Omitted defaultValidator
	m: map[MessageType]Validators{SimpleEventMessageType: {alwaysInvalidMsg()}})
err = msgv.Validate(Message{Type: CreateMessageType})
err != nil // True
```
</details>

<details>
<summary>Type of Change(s)</summary>

- Non-breaking Enhancement
- All new and existing tests passed.

</details>

<details>
<summary>Module Unit Testing: [PASSING]</summary>

</details>

<details>
<summary>PR Affecting Unit Testing: validator_test.go [PASSING]</summary>

```console
Running tool: /usr/local/bin/go test -timeout 30s -run ^(TestHelperValidators|TestMsgTypeValidator)$ github.com/xmidt-org/wrp-go/v3

=== RUN   TestHelperValidators
=== RUN   TestHelperValidators/alwaysInvalidMsg
--- PASS: TestHelperValidators (0.00s)
    --- PASS: TestHelperValidators/alwaysInvalidMsg (0.00s)
=== RUN   TestMsgTypeValidator
=== RUN   TestMsgTypeValidator/MsgTypeValidator_validate
=== RUN   TestMsgTypeValidator/MsgTypeValidator_validate/known_message_type
=== RUN   TestMsgTypeValidator/MsgTypeValidator_validate/unknown_message_type_with_provided_default_Validator
=== RUN   TestMsgTypeValidator/MsgTypeValidator_validate/known_message_type_with_a_failing_Validator
=== RUN   TestMsgTypeValidator/MsgTypeValidator_validate/unknown_message_type_without_provided_default_Validator
=== RUN   TestMsgTypeValidator/MsgTypeValidator_factory
=== RUN   TestMsgTypeValidator/MsgTypeValidator_factory/with_provided_default_Validator
=== RUN   TestMsgTypeValidator/MsgTypeValidator_factory/without_provided_default_Validator
=== RUN   TestMsgTypeValidator/MsgTypeValidator_factory/empty_list_of_message_type_Validators
=== RUN   TestMsgTypeValidator/MsgTypeValidator_factory/empty_value_'m'_map[MessageType]Validators
--- PASS: TestMsgTypeValidator (0.00s)
    --- PASS: TestMsgTypeValidator/MsgTypeValidator_validate (0.00s)
        --- PASS: TestMsgTypeValidator/MsgTypeValidator_validate/known_message_type (0.00s)
        --- PASS: TestMsgTypeValidator/MsgTypeValidator_validate/unknown_message_type_with_provided_default_Validator (0.00s)
        --- PASS: TestMsgTypeValidator/MsgTypeValidator_validate/known_message_type_with_a_failing_Validator (0.00s)
        --- PASS: TestMsgTypeValidator/MsgTypeValidator_validate/unknown_message_type_without_provided_default_Validator (0.00s)
    --- PASS: TestMsgTypeValidator/MsgTypeValidator_factory (0.00s)
        --- PASS: TestMsgTypeValidator/MsgTypeValidator_factory/with_provided_default_Validator (0.00s)
        --- PASS: TestMsgTypeValidator/MsgTypeValidator_factory/without_provided_default_Validator (0.00s)
        --- PASS: TestMsgTypeValidator/MsgTypeValidator_factory/empty_list_of_message_type_Validators (0.00s)
        --- PASS: TestMsgTypeValidator/MsgTypeValidator_factory/empty_value_'m'_map[MessageType]Validators (0.00s)
PASS
ok      github.com/xmidt-org/wrp-go/v3  0.303s

> Test run finished at 5/25/2022, 11:39:22 AM <
```

</details>

simplifies the usage of alwaysInvalidMsg

Return `ErrInvalidMsgTypeValidator` and `ErrInvalidMsgType` without additional details. We can add those error details downstream later

Thx @kristinapathak for the feedback! 🍻

already covered by "Not found success"

…ort & added examples

…om/denopink/wrp-go into denopink/FR-WRPValidationFramework

update examples

Doc/var update

Add test for `Nil default Validators` edge case

update examples output

doc update

Updates based on PR review

* exported alwaysValid
* decoupled data structures leveraging `Validators` to `Validator`
* added validateValidator

Add multierr to Validator

* converted remaining `Validators` to `Validator`
* remove func `validateValidator`
* move nil checks to `Validate` func
* remove Test struct
* add new unexported field to check if `TypeValidato` is valid
* add func `IsBad` to TypeValidato to say whether `TypeValidato` is valid
* update tests for testAlwaysInvalid and testAlwaysValid
* update tests names for `TestTypeValidator`
* add tests for `Validators`

* [misunderstanding] Remove unexported field `isbad` from `TypeValidator` and its references
* [misunderstanding] Remove `IsBad` func from `TypeValidator`
* Use `assert.Zero/NotZero` funcs to test `TypeValidator`'s state

… type is used

… `testAlwaysInvalid`

* rename `defaultValidators` to `defaultValidator`
* replace `defaultValidators` variadic to `defaultValidator Validator` in `NewTypeValidator` func