Add datasource for hosting channel

[modular-magician]

Add datasource for Firebase Hosting Channel. This is helpful for any existing channels created outsite of Terraform, such as the default "live" channel for any Hosting Site. Learn more

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

• 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#7079

[github-actions]

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