Teach TLSProxy how to parse CertificateRequest messages

We also use this in test_tls13messages to check that the extensions we
expect to see in a CertificateRequest are there.

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/9780)

(cherry picked from commit dc5bcb88d819de55eb37460c122e02fec91c6d86)