feat(runner): implement runner registration with gocast

[joschahenningsen]

Motivation and Context

As discussed, let's break down further development of the runner into smaller independent PRs.

Description

This PR implements the registration of the runner with gocast. This is achieved by:

• Cleaning up the gRPC services in runner.proto (FromRunner is nowRunnerManagerService)
• Creating a new package, runner_manager in gocast, which implements RunnerManagerService -> gocast is the runners manager
• Creating a new database model Runner with the fields hostname and port. The hostname is the primary key, on insert conflicts, the port is updated.
• On startup, the runner connects to this service to make itself known to gocast.

Steps for Testing

1. start gocast
2. start runner
3. observe logs
4. check database for inserted runner.

Screenshots