Add "ledgerIndex" field to some REST HTTP and MQTT API responses

[samuel-rufi]

There are endpoints in the API that return the "current" balance of an address or the state of an output.

For the caller it's hard to figure out what that "current" point in time was. Thats why an additional field was introduced to bind the balance or the output state to the milestone index at which the request was made. The index must be locked for the request. This sorting in time is especially helpful for balance reconcilation use-cases.

Example response:

{
  "data": {
    "addressType": 0,
    "address": "efdc112efe262b304bcf379b26c31bad029f616ee3ec4aa6345a366e4c9e43a3",
    "balance": 1338263,
    "dustAllowed": true,
    "ledgerIndex": 422153
  }
}

To summarize, the field should be added to the responses of following REST API endpoints:

/api/v1/addresses/{address}
/api/v1/addresses/ed25519/{address}
/api​/v1​/addresses​/{address}​/outputs
/api​/v1​/addresses​/ed25519/{address}​/outputs
/api/v1/outputs/{outputId}

Also, the field should be added to the relevant MQTT responses.

[samuel-rufi]

REST API calls are done, MQTT ones are missing