add github branch policy definition

[baywet]

example microsoft/kiota-http-guzzle-php#123

[MIchaelMainer]

You can use the https://github.com/MIchaelMainer/policyservicetoolkit to export existing settings. The suggested workflow is:

1. Export existing branch protection rules in the yaml file.
2. Check-in existing branch protection rules file at /.github/policies/.
3. Validate the rules are working as expected.
4. Update the branch protection rules in the yaml file and open PR.

Now you have gitop enabled your branch protection rules.