Add --quic_versions to QuicToyServer and improve version parsing This CL improves our version parsing code to allow using ALPN and parse a list, and uses that in a new --quic_versions flag on QuicToyServer. Note that ParseQuicVersionString is only used in tests and toy code. gfe-relnote: n/a, test-only PiperOrigin-RevId: 299245199 Change-Id: I33c0f73b6a094de0dba86b69bd598a24ee162872