Permit custom consumer registration to a session

Introduce the new API call lttng_register_consumer(...).

By giving a lttng_handle and a socket path, the fds will be sent to that
socket for the specific tracing session and domain. No kernel consumer
will be spawned by the session daemon for that session.

At this point, only kernel consumer is supported.

Signed-off-by: David Goulet <david.goulet@polymtl.ca>