cpp-common/bt2: add `bt2::PrivateQueryExecutor`

cpp-common/bt2: add `bt2::PrivateQueryExecutor`

A very straightforward wrapper of `bt_private_query_executor`.

There should exist an asConstQueryExecutor() method, but
`bt2::ConstQueryExecutor` doesn't exist yet as we don't need it to write
plugins. Therefore the loggingLevel() and isInterrupted() use
bt_private_query_executor_as_query_executor_const() internally.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I87c26e14afa4aa7937692aefa981213ad6a7f566
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11298
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>