[Feature] Support scp_from and scp_to for k8s charm

[chanchiwai-ray]

Description

It would be nice for python-libjuju to support scp_from and scp_to for k8s charm. For example, when running unit.scp_from, the follow error happen (in juju==3.2.4, libjuju==3.2.2):

  File "/snap/openstack/x10/lib/python3.10/site-packages/sunbeam/jobs/juju.py", line 467, in scp_from                   
    await unit.scp_from(source, destination)                                                                            
  File "/snap/openstack/x10/lib/python3.10/site-packages/juju/unit.py", line 333, in scp_from                           
    await self.machine.scp_from(source, destination, user=user,                                                         
AttributeError: 'NoneType' object has no attribute 'scp_from' 

So, it seems like those feature are not available for k8s charm. Related bug that was closed: #885

Urgency

Casually wishlisting

Code I'd Like to Run

unit.scp_from(source_in_the_container, destination_in_the_host, ...)
unit.scp_to(source_in_the_host, destination_in_the_container, ...)