Create modules to deploy runner onto GKE

Implements initial versions of the modules:

• k8s/operator
• k8s/runner

Removes the module:

• helm

This also brings in

• a scenario, which deploys a GKE cluster, installs the operator, and runs & registers a runner
• a example, showing the usage of the scenario

Notes:

• Does not expose many config knobs for the operator & runner module yet, we can deliberately open that up as we understand the need
• Adds an example of how to use the google/gke module in combination with the new modules
• There is a new "linter" which does some basic checks on the scenario documentation, however it has not been enabled in CI yet (not sure if we want to even go that route)

/xref: #82 (closed)