Bitbucket Trigger support for Manual, Pub/Sub and Webhook triggers

[modular-magician]

Adds field bitbucket_server_config to Trigger.git_file_source and Trigger.source_to_build, supporting manual, pub/sub and webhook triggers.

Also fixes TestAccCloudBuildTrigger_basic_bitbucket test.

fixes hashicorp/terraform-provider-google#15079

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).
• 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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Release Note Template for Downstream PRs (will be copied)

cloudbuild: added `git_file_source.bitbucket_server_config` and `source_to_build.bitbucket_server_config` fields to `google_cloudbuild_trigger` resource

Derived from GoogleCloudPlatform/magic-modules#8544