Introduce branch protection of main branch to require PR reviews

At present it appears that a PR author can merge their PR into a repo with no review from another reviewer. We should require that all PRs be reviewed by at least one additional engineer, and that this should be more than a rubber-stamping exercise - we should expect an adequate amount of time be allocated to ensuring correctness of implementation and API, before the approval is made.

To make this be an enforced expectation, we should [introduce branch protection](https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/defining-the-mergeability-of-pull-requests/managing-a-branch-protection-rule) in all of the SDK repos.