Add support for service return values

[ScottG489]

Coming up in the next release of HA (currently already available in beta), services will be able to return data. More information here:

home-assistant/architecture#777

I think it would make sense for the call service node to support returning this as part of it's output.

[zachowj]

#972

[zachowj]

Responses were added to call_service endpoint.

home-assistant/core@618b666