Add src.text.dmesg component class

Add src.text.dmesg component class

The new src.text.dmesg component class reads the standard input or a
specific file and converts each line to a CTF IR event, building the
appropriate metadata objects when needed.

This component does what babeltrace-log(1) used to do. The
babeltrace-log(1) application can be replicated as such:

    dmesg | babeltrace -c src.text.dmesg -p read-from-stdin=yes \
                       --no-debug-info -o ctf -w /path/to/output

The component class's initialization parameters are:

`read-from-stdin`:
    True to read the lines from the standard input stream (excludes
    `path`).

`path`:
    Path to the regular file from which to read lines (excludes
    `read-from-stdin`).

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