Firebase hosting channel

[modular-magician]

Add resource google_firebase_hosting_channel

If this PR is for Terraform, I acknowledge that I have:

• I opened one and commented that I would like to work on it (not necessary for very small changes) Support Firebase Hosting #12955.
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

`google_firebase_hosting_channel`

Derived from GoogleCloudPlatform/magic-modules#6791

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.