feature: Replace Idp redirectURL in authentication

[RRebekka]

Hi,

I wanted to ask if it would be possible to adjust the hostname of the r.Response.RedirectURL in the authentication method of the identity provider. In my setup I have an OIDC identity provider that is used in the Caddy security module. In the start up, I use the localhost:xxx address in the metadata url for fetching the meta data. Thus, the returned authorizationURL contains localhost as hostname. Later, when the user tries to authenticate themselves, they are redirected to the authorization backend via the localhost address. However, I want that the user is redirected to the authorization backend via the reverse proxied route of the authorization backend setup in Caddy. I can not use the reverse proxied route in the metadata_url, because the domain can change.

I have tested locally with my setup with something like the following, added to the Authenticate function in the idp/oauth/authenticate.go :

`

     var u, err = url.Parse(b.authorizationURL)
     b.logger.Debug("Authorization url")
     b.logger.Debug(b.authorizationURL)
     b.logger.Debug(u.Host)
     b.logger.Debug(u.Hostname())
     if u.Hostname() == "localhost" && err == nil {
	var u_, err_ = url.Parse(r.Upstream.BaseURL)
	if err != nil || err_ != nil {
		b.logger.Debug("Error in replacing url")
		return err
	}
	u.Scheme = "https"
	u.Host = u_.Host

	b.logger.Debug("Replaced url: ")
	b.logger.Debug(u.String())

	r.Response.RedirectURL = u.String() + "?" + params.Encode()
    } else {
	r.Response.RedirectURL = b.authorizationURL + "?" + params.Encode()
    }

`
Do you see any problems with that changes that I currently oversee?

Best regards,
Rebekka