BUG/MEDIUM: h3: parse content-length and reject invalid messages

BUG/MEDIUM: h3: parse content-length and reject invalid messages

Ensure that if a request contains a content-length header it matches
with the total size of following DATA frames. This is conformance with
HTTP/3 RFC 9114.

For the moment, this kind of errors triggers a connection close. In the
future, it should be handled only with a stream reset. To reduce
backport surface, this will be implemented in another commit.

This must be backported up to 2.6. It relies on the previous commit :
  MINOR: http: extract content-length parsing from H2