Allow parsing PROX packets with length prefix

The PROX version is special in that it has custom data after the first 9 bytes of the connection IDs. That means that trying to parse it with length prefix as an unknown QUIC version means that a random payload byte can be interpreted as the source connection ID length. This CL makes parsing succeed in that case, ignoring the source connection ID. This allows a previously existing test to pass.

gfe-relnote: allow parsing PROX packets with length prefix, protected by gfe2_reloadable_flag_quic_parse_prox_source_connection_id

Startblock:
  after 2019-09-24 08:00 in US/Pacific
PiperOrigin-RevId: 271678159
Change-Id: Ib079697029d7378cd7297e2400cefe8be33fad43