Introduce Legacy Version Encapsulation This feature allows modern QUIC versions to encapsulate their first flight in Q043 packets, to support SNI-extraction middleboxes that do not support modern versions. More details in the design document at <go/quic-legacy-encaps>. Introduce legacy version encapsulation, protected by gfe2_reloadable_flag_quic_dispatcher_legacy_version_encapsulation. PiperOrigin-RevId: 317921151 Change-Id: I2dd8a3641caf3fd3d283613de3d117c1bdbea632