MINOR: connection: Add a function to install a mux for a health-check

MINOR: connection: Add a function to install a mux for a health-check

This function is unused for now. But it will have be used to install a mux for
an outgoing connection openned in a health-check context. In this case, the
session's origin is the check itself, and it is used to know the mode, HTTP or
TCP, depending on the tcp-check type and not the proxy mode. The check is also
used to get the mux protocol if configured.