Support for Branch Protection bypass_pull_request_allowances

## New Feature

Can safe-settings add support for the "Allow specified actors to bypass required pull requests" feature of required pull request reviews for branch protection?

> * Is the functionality available in the GitHub UI? If so, please provide a link to information about the feature.

Available in the UI as:
```
Allow specified actors to bypass required pull requests
Specify people, teams, or apps who are allowed to bypass required pull requests.
```

> * Is the functionality available through the GitHub API? If the functionality is available, please provide links to the
API documentation (https://developer.github.com/v3/) as well as the Octokit documentation (https://octokit.github.io/).

Available in the [API](https://docs.github.com/en/rest/branches/branch-protection#update-branch-protection) as "bypass_pull_request_allowances".