shell: add new APIs to get the backend instances

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>