Add get_instance support for router

[sasameti]

Is your feature request related to a problem? Please describe.
when service multiple brokers using the router, the get_instance endpoint is not supported

Describe the solution you'd like
We should be able to get the service_instances

[eruvanos]

I agree on this.
The API does not provide the service ID in the get_instance call.

Without that information it is hard to figure out, which service broker should handle the request, without asking all of them.

Do you have a suggestion about an implementation?