cpp-common/bt2: `bt2::internal::SharedObj` -> `bt2::SharedObj`

cpp-common/bt2: `bt2::internal::SharedObj` -> `bt2::SharedObj`

`SharedObj` is not completely internal: it offers public methods,
including creation methods.

Also, we don't need to hide anything from the end user in this file.

Therefore make this class not internal.

Also apply to specific shared object aliases, for example
`bt2::SharedValue`.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I8bf1718a95de87b7c506ffab4868e63bc57a7391
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11367
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>