Improve channel reporting to list disabled ones if requested

Current situation

The /channels endpoint only lists known usable channels. SSH channels may have an invalid specification, but, except by checking the orchestrator logs, there is no way to identify them.

Also, unreachable agents are listed, with no way to know they are in fact unreachable.

Desired outcome

A way to list those disabled channels.

The channel manifest status.phase is extended to include a new UNREACHABLE value:

{
  "apiVersion": "opentestfactory.org/v1alpha1",
  "kind": "Channel",
  "metadata": {
    "name": string,
    "namespaces": string,
    "channelhandler_id": uuid
  },
  "spec": {
    "tags": [string, ...]
  },
  "status": {
    "lastCommunicationTimestamp": timestamp,
    "phase": IDLE or BUSY or PENDING or UNREACHABLE,
    "currentJobID": uuid or null
  }
}

IDLE means the channel is available for jobs. BUSY means the channel is currently processing a job (in which case currentJobID is the UUID of the job). PENDING means the channel has been offered for a job, but has not been selected (yet). UNREACHABLE means the channel is not assigned to a job but is not available: the orchestrator hasn't been able to communicate with it recently.