Add option to configure pre-defined "extraFields" to be added to each emitted thingEvent

[thjaeckle]

With the Ditto Signal Enrichment it is possible to dynamically configure for a single subscriber of "Thing Events" (whenever a Thing is modified) to get additional fields, which were not part of the change.
For example, all attributes or a special attribute which is required for the event consumer to have a context.

This works and is great.
However, this is also quite "costly" for Ditto - as the Gateway (for SSE and WebSocket) or Connectivity service of Ditto has to do another "internal" round-trip to fetch those extraFields. There are optimizations in place in order to cache them in a "smart cache" (which updates itself, etc.), but in the worst case, e.g. if the cache is full, this means a lot of roundtrips and networking.

There are scenarios when specific fields are always needed - that's why I suggest to configure (in the things service configuration) pre-defined extraFields which are pro-actively always sent in a DittoHeader field.
When the Connection or WebSocket "demands" this extraField (and e.g. no other), it can just use it from this header without additional roundtrips.
This should be configurable on a namespace level.

Example configuration:

pre-defined-extra-fields = [
  {
    namespaces = [
      "*"
    ]
    condition = "exists(definition)" # could be an option to also support a RQL condition here
    extra-fields = [
      "definition"
    ]
  },
  {
    namespaces = [
      "org.eclipse.ditto.lamps"
    ]
    extra-fields = [
      "attributes/manufacturer",
      "attributes/serial"
    ]
  }
]

One example use case behind this:

• We want to always get the definition of a Thing to "know" its current thing model (and semantic version of it)