[Feature Request] implement RPC endpoints to stop and start sources

### Is your feature request related to a problem? Please describe.

It would be helpful to have start and stop RPC endpoints for controlling sources from outside fluentd.
For example - if we have an external monitoring script - we could halt input using dynamic factors and resume input when appropriate.

### Describe the solution you'd like

implement RPC endpoints to stop and start sources

This was part of the original idea of implementing RPC endpoints.

See:
https://github.com/fluent/fluentd/issues/575
https://github.com/fluent/fluentd/pull/585

### Describe alternatives you've considered

N/A

### Additional context

N/A