rumqttd: Return list of topic subscribed on console API

[de-sh]

Expected Behavior

curl -X GET http://localhost:3333/subscriptions should return a response body containing JSON array listing all subscriptions.

Current Behavior

Console API returns nothing, but triggers a print to terminal, i.e. we are not constructing a response with requested info.

[swanandx]

currently all endpoints just print info ( except log filter update ) right?

This would mean after sending API request, we need a way to communicate to and from router and console.

Sending request to router is already what we are doing, we would need a way to get the response/stats from router and then send back as response.