Implement configurable OCSP responder

This would implement (9) from https://gist.github.com/sleevi/5efe9ef98961ecfb4da8. Presumably it would override the OCSP AIA in the cert if both are present.

Start with the referenced gist, especially item (9), then inspect the repository's certificate and OCSP handling to identify the relevant entry points. Done means the OCSP responder is configurable and overrides the certificate's OCSP AIA when both are present, with behavior covered by appropriate tests.

Written by the indexing model from the issue text.