Add a way to check whether token permissions are sufficient for a given operation

[Yiannis Triantafyllopoulos]

It would be good to have a way to validate that a token has all the right permissions i.e. repo and admin:org before it's used.

For GitHub: a response header X-OAuth-Scopes is returned that includes the current scopes https://docs.github.com/en/developers/apps/scopes-for-oauth-apps

For GitLab: an API call should return the current scopes https://docs.gitlab.com/ce/api/personal_access_tokens.html

[Yiannis Triantafyllopoulos]

Turns out that GitLab does not support an easy way to get the scopes of the current token. The above endpoint is not available in all flavours of GitLab and also it enumerates all the available tokens as opposed to the current one being used.

[Stefan Prodan]

We can't restrict the use of this library to GitLab Ultimate, I propose we don't implement the permission check for GitLab.

[Yiannis Triantafyllopoulos]

Ok I'll wait until the GitLab PR gets merged before I implement the method to return an ErrNoProviderSupport.