MINOR: ssl: add pattern fetch 'ssl_c_sha1'

MINOR: ssl: add pattern fetch 'ssl_c_sha1'

This new pattern fetch returns the client certificate's SHA-1 fingerprint
(i.e. SHA-1 hash of DER-encoded certificate) in a binary chunk.

This can be useful to pass it to a server in a header or to stick a client
to a server across multiple SSL connections.