Serves /.well-known/oauth-protected-resource (also under the endpoint's
path) and /.well-known/oauth-authorization-server. The protected
resource metadata of an endpoint on an external authorization server
(Sake.Auth.External) names that server; the authorization server
metadata is only Sake.Auth's, and so is
/.well-known/openid-configuration, OpenID Connect Discovery's, which
is there with openid: alone. Mount at the root:
forward "/.well-known", Sake.Auth.WellKnownor sake_well_known() in a Phoenix router.