The revision has no way for a server to put a request on a response stream,
so a handler that needs input from the user says so in its result instead: a
handler returning InputRequiredResult comes back as
resultType: "input_required" with the asks and an opaque requestState, and
the client retries the same request carrying inputResponses.
RequestContext::getInputContext() is what the handler reads them through.
RequestStateCodec signs and time-bounds the state, so a tampered or expired
one is refused rather than trusted - the state travels through the client,
which is not a place to keep something the server later believes.
The revision puts the method, the subject name and the mirrored arguments in
headers so an intermediary can route and cache without parsing the body. A
header that contradicts the body it travels with therefore has to be
refused, not ignored: StandardHeaderValidator compares each against the
request and answers -32020 on a mismatch.
Set with Builder::setHeaderValidator(); a transport with no header layer is
unaffected.
