Implement runner provisioning for a given repository

[marcofranssen]

By updating the protobuf message additional configuration parameters can be added.

By running make gen-proto the RPC client will be regenerated.

In agent/agent.go the logic can be implemented to iterate over the given set of repositories to implement the provisioning of a runner for a given repository.

This can be done via the os/exec package which can be used to invoke terraform cli or via https://terratest.gruntwork.io/.

The typical flow to provision a runner is:

• Create Runner registration token
• Provision the runner using terraform
• Keep track on the provisioned runner to be able to cleanup later (if runner doesn't clean itself) or if grace period expires to manage costs https://developer.github.com/v3/actions/self_hosted_runners/#remove-a-self-hosted-runner

Background info:
Best is if the Runner will auto unregister itself using https://developer.github.com/v3/actions/self_hosted_runners/#example-using-remove-token
This can be done by trapping some signals
https://github.com/joakimhellum/docker-actions-runner/blob/master/ubuntu-18.04/entrypoint.sh