cpp-common: add GCharUP

cpp-common: add GCharUP

Add GCharUP, a unique_ptr type that wraps a g_char pointer and has a
deleter that calls g_free.  It can be used to provide automatic memory
management of buffers returned by the GLib API.

Change-Id: I9d98be2f972cc008877b55a20eceeb3dce256cdb
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8170
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10842
Tested-by: jenkins <jenkins@lttng.org>
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>