Teach TLSProxy how to encrypt <= TLSv1.2 ETM records

Teach TLSProxy how to encrypt <= TLSv1.2 ETM records

Previously TLSProxy only knew how to "repack" messages for TLSv1.3.
Most of the handshake in <= TLSv1.2 is unencrypted so this hasn't been
too much of restriction. However we now want to modify reneg handshakes
which are encrypted so we need to add that capability.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>