Add GitHub workflow to enforce Google OSS license

[ava-cassiopeia]

Also adds the license to any places we missed.

[gemini-code-assist]

Code Review

This pull request successfully adds the Google OSS license header to a large number of files across the repository, which is a great step for compliance. The license headers are correctly formatted for the different file types.

However, the pull request title, 'Add GitHub workflow to enforce Google OSS license', suggests that a new GitHub workflow file should be part of this change. This file seems to be missing from the current set of changes. To fully achieve the goal of this PR, please consider adding the workflow file that enforces the presence of these license headers.

_{Note: Security Review is unavailable for this PR.}

[ava-cassiopeia]

However, the pull request title, 'Add GitHub workflow to enforce Google OSS license', suggests that a new GitHub workflow file should be part of this change. This file seems to be missing from the current set of changes. To fully achieve the goal of this PR, please consider adding the workflow file that enforces the presence of these license headers.

Gemini my dear, you reverse-hallucinated lol