feat(api domain): extend options for Get requests

[mildwonkey]

Description

Extends the optional configuration for API Domain Get methods:

• parameters (alternative to supplying in the URL)
• Headers
• Timeout
• Proxy configuration

I wanted to work through the PR review before I spend too much time on documentation so I opened separate docs tickets for each phase (I split up the main API Domain Optionality ticket by HTTP method to keep the PRs smallish).

Some notes of interest to reviews:

• I added a validation & mutation step that reads in the APISpec and normalizes some values, such as the timeout string to a time.Duration, and stores the normalized values in unexported fields in the APISpec struct. I then made GetResources a method on the ApiDomain so I didn't have to pass the Spec around between methods.
• Right now I'm treating ApiOptions in the Request as a full override of any top-level api options. I'm open to adding a flag to make that a merging behavior but wanted feedback before adding that (user or 🦄)

Related Issue

Fixes #765 (documentation is a separate PR)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Other (security config, docs update, etc)

Checklist before merging

• Test, docs, adr added or updated as needed
• Schema Updates applied
• Contributor Guide Steps followed

[meganwolf0]

(This was originally a comment about how something didn't work as expected, but then I realized I had the wrong spec structure 🤦‍♀️ )
Anyway, did some local testing and everything worked swimmingly!