MINOR: mux-h2/trace: add the default decoding callback

MINOR: mux-h2/trace: add the default decoding callback

The new function h2_trace() is called when relevant by the trace subsystem
in order to provide extra information about the trace being produced. It
can for example display the connection pointer, the stream pointer, etc.
It is declared in the trace source as the default callback as we expect
it to be versatile enough to enrich most traces.

In addition, for requests and responses, if we have a buffer and we can
decode it as an HTX buffer, we can extract user-friendly information from
the start line.