Automate the creation of branches/PRs for new Maps SDK releases

[friedbunny]

For consistency and ease, we should automate the creation of testing branches for new Maps SDK releases here.

Rough outline

1. Maps SDK (pre-)release is finished and pod is available.
2. Some trigger is sent to this repo, kicking off a CI job here.
3. CI job looks for a new SDK release, then:
   • Creates a new commit bumping to that release.
   • Pushes to the ios-vX.X.X branch.
   • Creates a PR, if none already exists.

/cc @mapbox/maps-ios

[friedbunny]

Once #167 is addressed, this can also kick off a new TestFlight build for a given SDK release.