Support auth via certificates and headers

[devon-mar]

This PR adds support for authentication via a client certificate (#300) or via a header set by a reverse proxy.

In order to use a client certificate (or auth by header), the username passed to docker login must match that of the certificate's CN or the value in the header used for authentication. The password doesn't matter. I've added an example to show how I'm using it in the examples folder.

[hmoffatt]

Thanks for this PR. This means I can use for example mod_auth_openidc for Apache2 to handle authentication.

[Leobaillard]

Can this feature be merged? It would be very useful!

[FootStark]

+1 Would love to see this merged to enable "X-Forwarded-User"-Auth via reverse proxy!