feat: Add Pub/Sub Schema support for changing definitions via revisions

[modular-magician]

Allows schema definition to be changed by committing schema revisions. Fixes hashicorp/terraform-provider-google#13997. This replaces GoogleCloudPlatform/magic-modules#7981. @SarahFrench and @melinath have the most context on this.

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• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• 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).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
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• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

pubsub: Allowed `definition` of `google_pubsub_schema` to change without deleting and recreating the resource by using schema revisions (https://cloud.google.com/pubsub/docs/schemas#commit-schema-revision)

Derived from GoogleCloudPlatform/magic-modules#8079