ServiceConfig for auth ?

[kozak]

Hi, Thanks for a great library!
I've wanted to override the auth endpoint urls for testing/mocks. Gogol provides this ability for services via ServiceConfig. I was thinking of adding a similar feature to Auth. Would this be ok, or would you prefer a different way for overriding tokenRequest and accountsRequest (https://github.com/brendanhay/gogol/blob/develop/gogol/src/Network/Google/Internal/Auth.hs#L261-L287)? Cheers!