setup repo required checks in .github/sync-repo-settings.yaml

[zhumin8]

Created as a separate issue from discussion on https://github.com/googleapis/sdk-platform-java/issues/2733.

Also setup protection for protected branches, similar to google-cloud-java here

Note that sync-repo-settings will override existing required checks via settings. current required checks: https://screenshot.googleplex.com/83qzNAk35tADCmr.png