ctf.fs source: add `get-metadata-info` query info action

ctf.fs source: add `get-metadata-info` query info action

The `get-metadata-info` query info action of the ctf.fs source returns,
if successful, the (possibly decoded) metadata string and if the
metadata file is packetized or not. It only needs the `path` parameter
which is the trace directory path (non-recursive).

You can test it with the query-info command:

    babeltrace query-info --source ctf.fs get-metadata-info
                          -p 'path="/path/to/trace"'

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>