Endpoint for retrieving the current configuration from the airy-controller

[ljupcovangelski]

We need an endpoint on the airy-controller from where we can retrieve the current configuration from the Airy Core instance.

The following should be made possible on the airy-controller side:

curl -X GET -H "Content-Type: application/json" http://airy.core/controller/config

Then we need to implement this in the Airy CLI. When running airy config we should write the configuration to an airy.yaml file in the workspace.

Related to #2756

[ljupcovangelski]

Related to the authentication issue, the URI of the service for now should be cluster.get.

curl -X GET -H "Content-Type: application/json" http://airy.core/cluster.get

cluster.apply will be used for applying the configuration.

[ljupcovangelski]

The CLI needs to access the information from the endpoint and not through the Kubernetes API.
This is because we want to make the CLI independent of Kubernetes.

[ljupcovangelski]

Done.