Automate configuration of branch protections

[scpeters]

When our CI job names change, it requires significant manual effort by a maintainer with significant github privileges to update the branch protection settings (at a URL like https://github.com/gazebo-tooling/release-tools/settings/branches), for example in #1416.

Performing this task manually is even a bit tricky as you can only mark CI jobs as required if they have executed in the past week. Since we may have different CI job names depending on the target branch, you may need to open a pull request to trigger a job in order to mark it as required for a branch.

Some discussion about automating configuration of branch protections is discussed in cli/cli#3528, making use of the branch protection REST API.

[azeey]

Thanks for creating the issue @scpeters. There's ongoing work by @j-rivero to use Terraform to accomplish this, but we didn't have an issue to track it.