support OAUTH_CLIENT_ env in all implementation
support OAUTH_CLIENT_ env in all implementation

preserve custom client_{id|secret}_env for backward-compatibility,
but let's move to removing the distinction in the docs.

jupyter/oauthenticator,enolfc/oauthenticator,yuvipanda/mwoauthenticator,NickolausDS/oauthenticator,minrk/oauthenticator,yuvipanda/mwoauthenticator,maltevogl/oauthenticator,jupyterhub/oauthenticator,jupyter/oauthenticator