Implement PKCE for OAuth2

[lf-]

Hi!

OpenGist currently doesn't support PKCE for OAuth2, the lack of which is not super secure. I have gone looking at the underlying library and it does support it since markbates/goth@7593a57 which this project has, but implementing it is nonobvious, but here are some relevant issues/prs:

markbates/goth#516
go-gitea/gitea#21426

Additionally, here is some code, but it seems to be based on an older version of the goth code prior to direct pkce support: https://github.com/mozilla/protodash/blob/cdfb39b44c1bd8fe9d256c97d892b9fd37c88103/pkce/session.go#L43