fix(auth): app_id can be an str

[shtrom]

GitHub accepts (and recommends) Client ID strings when generating tokens [0].

[0] https://docs.github.com/en/apps/creating-github-apps/authenticating-with-a-github-app/generating-a-json-web-token-jwt-for-a-github-app#:~:text=Use%20of%20the%20client%20ID%20is%20recommended%2E

fixes: #145

[bhearsum]

I think the reason this is typed as an int, is because all valid github app ids are ints AFAIK, even though it goes over the wire as a string in the end, so only accepting ints is a form of validation that callers are doing the right thing.

With that said, I imagine that this is often is read by callers from places that are strings...and it can be annoying to cast just to have the value cast back to a string internally.

@ahal - maybe you can tiebreak this?

[shtrom]

My app_id is very much not a int (:

Correction: my App ID is indeed an int, but my Client ID isn't. The GitHub endpoint for token generation accepts both, but recommends the use of the (str) Client ID https://docs.github.com/en/apps/creating-github-apps/authenticating-with-a-github-app/generating-a-json-web-token-jwt-for-a-github-app#:~:text=Use%20of%20the%20client%20ID%20is%20recommended%2E

[shtrom]

Maybe we should rename that parameter to client_or_app_id, but that may not be backward compatible.

EDIT: I updated the docstring for app_id to reflect the polymorphism and GitHub's recommendation.