[Generic] Add support for client_secret_post method

[enolfc]

Support the client_secret_post method as specificed at https://openid.net/specs/openid-connect-core-1_0.html#ClientAuthentication

Keeps the behaviour of the basic_auth config and adds a new config parameter client_auth_method to support client_secret_post and client_secret_basic (this is the currently existing one)

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[enolfc]

This is very similar to #274

[GeorgianaElena]

Thank you very much for this work @enolfc! This and the discussion in the meta issue has been really helpful in finding and fixing this bug ✨

I will close this PR now that the bug has been fixed as part of https://github.com/jupyterhub/oauthenticator/pull/568 and this refactor https://github.com/jupyterhub/oauthenticator/pull/526