Add support for adding apps to branch protection

[gposton]

This PR adds support for adding apps to branch protection rules.

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[gposton]

@googlebot I signed it!

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[gmlewis]

Is this related to #1275 and #1283 ?
If so, then I think this issue is already assigned and someone is working on it, and we need to close this.

In general, in this repo we try to make it clear who is working on what issues so that work is not duplicated. As a result, PRs should in general be associated with open issues.

Thanks for your understanding, @gposton.

[gposton]

Yes, this addresses those issues as well.

[gmlewis]

Closing this issue as it was resolved in #1283.