MINOR: hq-interop: add request start-line traces

MINOR: hq-interop: add request start-line traces

Add traces to log the start-line of received (FE side) or sent (BE)
requests.

This uses a similar pattern with already supported HTTP/3 header traces.
However, this only requires minimal trace verbosity. This is because
these traces will be mostly useful for QUIC interop testing. However it
is probably not desirable to use advanced verbosity in this context to
avoid increasing the traces output.