Add the ability to mutate TLS handshake messages before they are written

Add the ability to mutate TLS handshake messages before they are written

We add callbacks so that TLS handshake messages can be modified by the test
framework before they are passed to the handshake hash, possibly encrypted
and written to the network. This enables us to simulate badly behaving
endpoints.

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20030)