Explore requirements for full PKI handling

[stan-janssen]

During one of our LF Energy Webinars, a question came up regarding PKI support in OpenLEADR.

At the moment, we support adding a custom CA (root) file that all certificates must be validated against. This should (by default) be the OpenADR Alliance's CA file.

I'd like to make a list of things that we still need to add to make this more complete.

• [ ] Add functionality that can look up a certificate in a CRL to see if it has been revoked