Supports a GraphQL query to retrieve the status of a Forwarder session for a device

[davidebriani]

As a complement to issue #445, API clients may need to retrieve the status of a Forwarder session they requested before attempting to access it.

The Edgehog backend can support a forwarderSession GraphQL query.
A client would use the query to inspect the status of a device → Forwarder session, and know whether the session is ready to be accessed or if it doesn't exist yet.

query {
  forwarderSession(deviceId: ID!, sessionToken: String!) {
    token: String
    status: "CONNECTED" | "CONNECTING"
    secure: Boolean
    forwarderHostname: String
    forwarderPort: Integer
  }
}

To support the query, the backend queries the Astarte interface ForwarderSessionsState for the specified device and returns the details about the session identified by the sessionToken.

When the device is not managing the session yet (or anymore), the session effectively doesn't exist and the query returns an error.