QUIC Wire Encoding: Support Retry Integrity Tag Calculation

QUIC Wire Encoding: Support Retry Integrity Tag Calculation

This adds support for calculating and verifying retry integrity tags. In
order to support this, an 'unused' field is added to the QUIC packet
header structure so we can ensure that the serialization of the header
is bit-for-bit identical to what was decoded.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/19703)