Restart model service process without reloading the container

## Motivation  

In early bootstrap phases, model service needs to be restarted frequently to find out the working and performant parameter. In such cases following the original model service restart - terminating and creating a fresh new session - would be a bottleneck, especially when compared to just restarting the inference framework process itself. By extending current kernel runner spec a little bit mode, we can think achieve restarting the model service process itself while not touching the container lifecycle.

## Required Features

- Create a new REST API to trigger restarting the model service process
- Create a new RPC Handler to handle model service process restart request
- Add a new kernel runner interface which shuts down ongoing model service process

## Impact  

- [Backend.AI](http://Backend.AI) Agent: new RPC interface
- [Backend.AI](http://Backend.AI) Manager: new REST API

## Testing Scenarios  

- Trigger restart of the model service process and check if the inference worker loads like before
- Reiterate step 1, this time modifying the `model-definition.yaml` file. Check if the change is reflected to the runner.

JIRA Issue: BA-441