Added some new APIs to get the backend instances more easily,
so that dev does not need to rely on `shell_backend_*_get_ptr`,
which looks more like a hack to access a local variable.
These APIs are basically copied from the log backend
implementation.
Added testcase for the APIs.
Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Yong Cong Sin