Add new utility functions for authenticating as a Github App

[yorinasub17]

Description

This adds a new package github that contains functions for interacting with GitHub. To start, this contains functions that are useful for authenticating as Github App, which is becoming increasingly common with our Lambda based services (refarch-deployer-runner, discussion-labeler, and soon release-notes-drafter). Once this is released, I'll start updating each of these to use this central implementation.

TODOs

Please ensure all of these TODOs are completed before asking for a review.

• [x] Ensure the branch is named correctly with the issue number. e.g: feature/new-vpc-endpoints-955 or bug/missing-count-param-434.
• [x] Update the docs.
• [x] Keep the changes backward compatible where possible.
• [x] Run the pre-commit checks successfully.
• [x] Run the relevant tests successfully.
• [x] Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.

[yorinasub17]

Thanks for review! Merging now.