Add placeholder for accessing thing values in connections

[thjaeckle]

Currently, connections can make use of various placeholders to e.g. construct the "address" (e.g. a URL) to invoke.

Example of a "http-push" connection target:

{
  ...
  "targets": [{
    "address": "POST:/my-endpoint/{{thing:name}}/foo/{{header:bar}}",
    "topics": ["_/_/things/live/messages"],
    "authorizationContext": ["integration:ditto"],
    "issuedAcknowledgementLabel": "live-response",
    "headerMapping": {}
  }],
  ...
}

What however is not yet possible (without specifying a custom JavaScript payload mapper), is to access e.g. metadata of the thing to construct the endpoint.

For example, if the current weather for a "thing" shall be retrievable by sending a message to the digital twin, it would be very convenient to just be able to configure:

{
  ...
  "targets": [{
    "address": "GET:/weather?longitude={{thing-json:attributes/location/lon}}&latitude={{thing-json:attributes/location/lat}}",
    "topics": ["_/_/things/live/messages?filter=eq(topic:subject,'retrieve-weather')&extraFields=attributes/location"],
    "authorizationContext": ["integration:ditto"],
    "issuedAcknowledgementLabel": "live-response",
    "headerMapping": {}
  }],
  ...
}

Assuming the thing contains the longitude and latitude in its attributes like:

{
  "thingId": "bum:lux",
  "attributes": {
    "location": {
      "lon": -77.0364,
      "lat": 38.8951
    }
  }
}

One could simply retrieve the weather of the twin by invoking:

POST /api/2/things/bum:lux/inbox/messages/retrieve-weather

In order to achieve that, we "simply" need a new Placeholder resolver, being able to access the thing's payload (also via extraFields).