channel: Remove invalid free_channel call

When a job fails to start, it will already call the exit_cb which takes care of
freeing the channel.
runtime: implement python/clipboard with runtime scripts

- Move the python provider implementation(script_host.py) from the python-client
  repository to this one
- Use the autoload scripts to implement python/clipboard features
- Add runtime scripts for working with external plugins and hosts
- Add tests for the new runtime scripts