Move the QUIC_CONNECTION typedef to internal headers

Move the QUIC_CONNECTION typedef to internal headers

Also add internal functionality to get a QUIC_CONNECTION pointer from
an SSL pointer, and setters / getters for the GQX and ACKM fields.

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18838)