[Server][Tests] Complete the multi round-trip request flow (SEP-2322) (#446)

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.

tests/Conformance/Fixtures/docker-compose.yml

@@ -2,8 +2,7 @@ services:
   nginx:
     image: nginx:1.26-alpine
     ports:
-      # Overridable so the fixture can coexist with whatever else a developer
-      # already has on 8000; CI and the Makefile default keep using 8000.
+      # Overridable so the fixture can coexist with anything already on 8000.
       - "${CONFORMANCE_PORT:-8000}:80"
     volumes:
       - ./nginx.conf:/etc/nginx/conf.d/default.conf:ro