Ability to override authorization end-point for OIDC

[SaurabhAZip]

Hello All,
I am using dex as identity service for multiple Idp's (google and github)

Dex provides discovery URL as example.com and
github auth url - example.com/auth/github
google auth url - example.com/auth/google

Now when we are using step-ca server to generate short-lived certificate, browser window opens the default discovery url - example.com and user has to choose one of the IdP.

For our use case, i would like to specify one Idp.
E.g. configurationEndPoint - example.com
authEndPoint-example.com/auth/github

But there is no way to add provisioner with --authorization-endpoint option.

Although interestingly,
step oauth
already provides the
--authorization-endpoint
option

I am happy to make changes and create a PR for the changes if there are no objections.