Add method to get the server certificate

[Leont]

For purposes of channel binding (when SCRAM authenticating), it would be necessary for me to be able get my hands on the server certificate (as a blob, I need to hash it).

[Leont]

Or actually, I do need to know the hash associated with the signature algorithm too.

[jnthn]

I don't expect to have time for any feature development on this module for the foreseeable future, but will happily review a PR for this.

[Zer0-Tolerance]

Hello Jonathan,
I would also need the same feature to get the all the server cert info, not sure how to do it otherwise I would be happy to work on a PR.