feat(release-kit): add use firebase-functions RC checkbox and validation

[inlined]

Description

Adds a use_firebase_functions_rc checkbox input to the Release Kit workflow (release-kit.yaml).

Changes

• Adds use_firebase_functions_rc boolean input (defaults to false).
• Adds validation in both test and release jobs ensuring that use_firebase_functions_rc can only be set when is_prerelease is true.
• Runs npm install --save firebase-functions@next during dependency installation in both the test and release jobs when the option is enabled.
• Displays Functions RC configuration in the Dry Run Summary.

Testing

• Validated YAML syntax.
• Validated workflow with zizmor.
• Verified validation logic matrix for all combinations of use_firebase_functions_rc and is_prerelease.

[gemini-code-assist]

Note

Gemini is unable to generate a review for this pull request due to the file types involved not being currently supported.