Add GitHub followers endpoint

[SethFalco]

Support the followers endpoint, I'm hoping this can be generalized for both users and organizations, but we'll see.

Use cases include tracking the growth of a users/orgs followers, and visualizing what organizations those users are from. (i.e. to track how many users in FANG organizations are following the project)

Reference:

• https://docs.github.com/en/rest/users/followers

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(I can probably contribute this one myself, will check the docs later!)