chore: Add experimental API annotation

[mattcreaser]

• PR title and description conform to Pull Request guidelines.

Issue #, if available:

Description of changes:
Adds a new annotation @ExperimentalAmplifyApi that we can use when creating new APIs without committing to full stability.

This follows the pattern of many other libraries, such as @ExperimentalCoroutinesApi, @ExperimentalComposeApi, @ExperimentalAnimationApi, etc.

How did you test these changes?
n/a

Documentation update required?

• No
• Yes (Please include a PR link for the documentation update)

General Checklist

• Added Unit Tests
• Added Integration Tests
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• Ensure commit message has the appropriate scope (e.g fix(storage): message, feat(auth): message, chore(all): message)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.22%. Comparing base (5410c35) to head (8da7405).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3227      +/-   ##
==========================================
- Coverage   55.23%   55.22%   -0.02%     
==========================================
  Files        1076     1076              
  Lines       31886    31886              
  Branches     4749     4749              
==========================================
- Hits        17613    17609       -4     
- Misses      12448    12450       +2     
- Partials     1825     1827       +2     

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