Migrate and getsystem parameter fix

[j91321]

Fixes error that occurs when using the migrate and getsystem. It seems that it is required to use CopyFrom instead of assignment when using protobuf objects into requests.

Tested both commands, and they seem to work with this fix.

Example of the error that is raised when using assignment:

<class 'clientpb.client_pb2.ImplantConfig'>
Traceback (most recent call last):
  File "test-sliver.py", line 36, in <module>
    asyncio.run(main())
  File "/usr/lib/python3.8/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/usr/lib/python3.8/asyncio/base_events.py", line 616, in run_until_complete
    return future.result()
  File "test-sliver.py", line 25, in main
    result = await session.get_system("spoolsv.exe", implantConfig)
  File "/mnt/c/Users/admin/Documents/work/venv-plugin-dev/lib/python3.8/site-packages/sliver/interactive.py", line 264, in get_system
    system.Config = config
AttributeError: Assignment not allowed to field "Config" in protocol message object.