Add the ability to mutate QUIC packets before they are written

Add the ability to mutate QUIC packets before they are written

We add callbacks so that QUIC packets can be modified by the test
framework before they are 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)