Implement a base GraphQL stream to reduce unnecessary API calls

[Ry-DS]

Currently we have a REST base stream implemented for our tap:
https://github.com/MeltanoLabs/tap-github/blob/main/tap_github/client.py#L10

However, it would be greatly beneficial if we also considered making a base stream for the Github GraphQL API. This is because the Graphql API uses a 'credits' system which makes it a lot more adaptive at collecting granular data compared to REST where each resource, no matter how small, takes up an entire request.

Streams that can greatly benefit from the GraphQL API:

• PR Files stream (WIP in Add PR Files stream #45)
• PRs in general Add missing fields to PR stream #49