Extend the rotation API to provide network trace archive locations

The current lttng-ctl rotation API does not allow a user to
differentiate between a network or local trace archive location.

The API currently only provides a "path" which is absolute when
a local rotation is completed, and relative (to an unknown location)
when the trace is streamed to a relay daemon.

This change introduces the lttng_trace_archive_location interface
to express these locations unambiguously. It is currently only
used by the rotation control API, but the intention is to also use
it for future interfaces which need to express a location description.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>