test: Fix nondeterminism in interleaved notifications test

Protect `rpcnotify` call in a condition to avoid having a notification leak into
other tests.
runtime: ProviderCall implementations

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