feat(transport): make max_udp_payload_size tparam configurable
Some QUIC servers use an initial MTU larger than 1280 bytes. This is problematic
when connecting to such servers via a proxy (e.g. MASQUE connect-udp). While
most unproxied paths can handle >1280 bytes, some proxied paths cannot.
Firefox will need to restrict the max_udp_payload_size to 1232 bytes (i.e. 1232
+ 40 (v6) + 8 (UDP)) on proxied connections to support such restrictive paths.