Support union types

[revati]

Describe the bug
Union types are not rendered as inputs in form

To Reproduce

defmodule Resource.PathElement do
  use Ash.Type.NewType,
    subtype_of: :union,
    constraints: [
      types: [
        integer: [type: :integer],
        string: [type: :string]
      ]
    ]
end

Expected behavior
Have input

Runtime

• Ash admin version: 0.10.10-rc.1

Additional context

[error] GenServer #PID<0.5962.0> terminating
** (RuntimeError) Got no "_union_type" parameter, and no union type had a tag & tag_value pair matching the params.

If you are adding a form, select a type using params: %{"_union_type" => "type_name"}, or if one
or more of your types is using a tag you can set that tag with params: %{"tag" => "tag_value"}.

Params:

nil

Available types:

[integer: [type: :integer], string: [type: :string]]

(ash_phoenix 2.0.0-rc.8) lib/ash_phoenix/form/auto.ex:263: AshPhoenix.Form.Auto.determine_type/3
(ash_phoenix 2.0.0-rc.8) lib/ash_phoenix/form/auto.ex:155: anonymous fn/5 in AshPhoenix.Form.Auto.unions/3
(ash_phoenix 2.0.0-rc.8) lib/ash_phoenix/form/form.ex:3156: AshPhoenix.Form.update_opts/3
(ash_phoenix 2.0.0-rc.8) lib/ash_phoenix/form/form.ex:1213: anonymous fn/9 in AshPhoenix.Form.validate_nested_forms/6
(elixir 1.16.2) lib/enum.ex:2528: Enum."-reduce/3-lists^foldl/2-0-"/3
(ash_phoenix 2.0.0-rc.8) lib/ash_phoenix/form/form.ex:1210: anonymous fn/7 in AshPhoenix.Form.validate_nested_forms/6
(elixir 1.16.2) lib/enum.ex:2528: Enum."-reduce/3-lists^foldl/2-0-"/3
(ash_phoenix 2.0.0-rc.8) lib/ash_phoenix/form/form.ex:1058: AshPhoenix.Form.validate/3
(ash_phoenix 2.0.0-rc.8) lib/ash_phoenix/form/form.ex:1279: anonymous fn/9 in AshPhoenix.Form.validate_nested_forms/6
(elixir 1.16.2) lib/enum.ex:2528: Enum."-reduce/3-lists^foldl/2-0-"/3
(ash_phoenix 2.0.0-rc.8) lib/ash_phoenix/form/form.ex:1210: anonymous fn/7 in AshPhoenix.Form.validate_nested_forms/6
(elixir 1.16.2) lib/enum.ex:2528: Enum."-reduce/3-lists^foldl/2-0-"/3
(ash_phoenix 2.0.0-rc.8) lib/ash_phoenix/form/form.ex:1058: AshPhoenix.Form.validate/3
(ash_phoenix 2.0.0-rc.8) lib/ash_phoenix/form/form.ex:1279: anonymous fn/9 in AshPhoenix.Form.validate_nested_forms/6
(elixir 1.16.2) lib/enum.ex:2528: Enum."-reduce/3-lists^foldl/2-0-"/3
(ash_phoenix 2.0.0-rc.8) lib/ash_phoenix/form/form.ex:1210: anonymous fn/7 in AshPhoenix.Form.validate_nested_forms/6
(elixir 1.16.2) lib/enum.ex:2528: Enum."-reduce/3-lists^foldl/2-0-"/3
(ash_phoenix 2.0.0-rc.8) lib/ash_phoenix/form/form.ex:1058: AshPhoenix.Form.validate/3
(ash_phoenix 2.0.0-rc.8) lib/ash_phoenix/form/form.ex:985: AshPhoenix.Form.validate/3
(ash_admin 0.10.10-rc.1) lib/ash_admin/components/resource/form.ex:1275: AshAdmin.Components.Resource.Form.handle_event/3

[revati]

Related to #71