cpp-common/bt2: add C++ plugin bindings

Add C++ bindings around `bt_plugin`.  There is only the const version
(`ConstPlugin`), because there is no need for a non-const plugin.

Get the scalar properties of a plugin using the `name`, `description`,
`author`, `license` and `path` methods.

Get the version of a plugin using the `version` method.

Get the component classes provided by the plugin using the
`sourceComponentClasses`, `filterComponentClasses` and
`sinkComponentClasses` methods.  These methods return proxy objects that
can be iterated on.  They also defined an `operator[]` method to access
component classes by name.

Change-Id: I5121c7c54a8058e5ca116618472ec8bb63f3a825
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11822
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>