ci: unmanaged dependency check

[suztomo]

This check avoids accidentally adding 3rd party dependencies. This is one of the items listed in go/cloud-sdk-java-dependency-governance-design. Sydney has reviewed this.

b/320678476

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• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
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• Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> ☕️

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[suztomo]

Unmanaged dependency check / unmanaged_dependency_check (pull_request) Successful.

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• .github/sync-repo-settings.yaml

[suztomo]

@sydney-munro Hi Sydney, this is the one of the steps in go/cloud-sdk-java-dependency-governance-design. Would you review this?