Support wildcard in "fields" selectors in order to receive all "desired" properties of all features of a thing

[thjaeckle]

As part of #696, but also as a general useful enhancement of Ditto, the "fields" selector which can be used to perform partial requests with field selector shall support the wildcard _ (similarly to MQTT, targeting only 1 level) in order to define "anything" for a certain JSON pointer segment.

In scope of the "desired" project

Such queries are needed in order to get all the (unapplied) "desiredProperties" of a thing, e.g. with such a query:

GET /api/2/things/org.eclipse.ditto:my-thermostat-1?fields=features/_/desiredProperties

When using the search API, also all "desiredProperties" fields may be selected as result of the search:

GET /api/2/search/things?filter=like(attributes/manufacturer,"ACME*")&fields=features/_/desiredProperties

In general

That is very useful when feature names of a thing are unknown and special parts of all features shall be retrieved with one command.

Example: For the given thing JSON:

{
  "thingId": "org.eclipse.ditto:traffic-light",
  "policyId": "org.eclipse.ditto:traffic-light",
  "features": {
    "red-lamp": {
      "properties": {
        "on": true,
        "color": "red"
      }
    },
    "yellow-lamp": {
      "properties": {
        "on": true,
        "color": "yellow"
      }
    },
    "green-lamp": {
      "properties": {
        "on": false,
        "color": "green"
      }
    }
  }
}

The goal is to get only the "on" properties of all lamps of a traffic light.

That could be done with the following HTTP request:

GET /api/2/things/org.eclipse.ditto:traffic-light?fields=features/_/properties/on

[thjaeckle]

This should be already available via the recently done PR #1277

Syntax:

GET /api/2/things/org.eclipse.ditto:my-thermostat-1?fields=features/*/desiredProperties

GET /api/2/things/org.eclipse.ditto:traffic-light?fields=features/*/properties/on