upstream HTTP/3 0-RTT should work, and be restricted to safe methods #18715
Description
Section 4 of RFC 8470 says:
Absent other information, clients MAY send requests with safe HTTP methods ([RFC7231], Section 4.2.1) in early data when it is available and MUST NOT send unsafe methods (or methods whose safety is not known) in early data.
In Envoy we should make sure that we only send 0-RTT for safe methods: GET, HEAD, OPTIONS, and TRACE.