Add client SSL certificate and key support

This commit adds basic client certificate and key support to rex'
SslTcp sockets. This includes building params and getting the cert
material using the same logic as the standard SslCertificate param.
Ideally raw key material should be passed to these init methods
when possible to enable sockets to operate on key material
extracted via meterpreter on the fly, without writing files locally.

The next logical steps are to add validation to SslTcp, add child
certificate validation to the Ssl servers, and a simple API for
generating child certificates from existing servers.