Add support for manual builds, git source for webhook/pubsub triggered builds and filter field

[modular-magician]

Fixes #10779
Fixes #10434
Fixes #10387
Fixes #9755
Fixes #8374
Fixes #9189
Fixes #9883

Add support for REST fields sourceToBuild and gitFileSource which adds support for manual builds as well as adds support for webhook and pubsub triggered builds with source code.
Also added support for filter field that is used with webhook and pubsub triggered builds.
Also added examples for webhook, pubsub (with filter) and manual build.

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

• 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).
• 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)

cloudbuild: Add support for manual builds, git source for webhook/pubsub triggered builds and filter field

Derived from GoogleCloudPlatform/magic-modules#5738

[github-actions]

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