bt2: add `ComponentDescriptor` class and test

bt2: add `ComponentDescriptor` class and test

The goal of a `ComponentDescriptor` object is to eventually be used
within an array of descriptors passed to a function which computes an
effective message interchange protocol version with the help of version
support component class methods.

I choose not to wrap the `bt_component_descriptor_set` API as this is
so simple and will be used at a single location.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ifef9b4fb0a7cf0278dbf8bed2d18d0b47934a272
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1873
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>