allow using github app auth for private repos

[KyriosGN0]

in order to clone private repos with github app we need to generate JWT and sign get access token for the clone command
im happy to work on a PR, but would like some direction on how to implement (where do we generate the JWT/ how often etc)

[Greyeye]

this may be a good start
https://gist.github.com/rajbos/8581083586b537029fe8ab796506bec3#file-github-app-jwt-sh

https://github.com/golang-jwt/jwt
is a JWT lib that is often in use by many (in fact, I dont know any other jwt libs)

The generated token should only be valid as long as you need to access the API (seconds, not days), so I'd say it should be valid for a few minutes. Ideally, middleware should be written so that GitHub transport can check/generate a new token on the fly.

[KyriosGN0]

@Greyeye do you think its best to to just check in the clone function if the repo is private?
that would mean extending the repo struct to be aware of this property ?