QUIC Dispatch: Introduce the QUIC_XSO object

QUIC Dispatch: Introduce the QUIC_XSO object

The QUIC_XSO (external stream object) is to a QUIC stream what a
QUIC_CONNECTION is to a QUIC connection. Both are SSL objects. The
QUIC_CONNECTION type is the internal representation of a QUIC connection
SSL object (QCSO) and the QUIC_XSO type is the internal representation
of a QUIC stream SSL object (QSSO) type. The name QUIC_XSO has been
chosen to be distinct from the existing QUIC_STREAM type which is our
existing internal stream type. QUIC_XSO is to a QUIC_STREAM what
QUIC_CONNECTION is to a QUIC_CHANNEL; in other words, QUIC_CONNECTION
and QUIC_XSO objects form part of the API personality layer, whereas
QUIC_CHANNEL and QUIC_STREAM objects form part of the QUIC core and are
distinct from the API personality layer.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20765)