Create an interface for processors

We should create multiple interfaces for this.

There should be a ProcessorInterface and a ProcessorPluginBase that implements that interface. In the ProcessorInterface there should be a configureForm to edit settings in the UI.

We should also copy the settings from the search api processors where they have stages in the configuration of a Processor:

 *   stages = {
 *     "pre_query" = 0,
 *     "post_query" = 0,
 *     "pre_build" = 0,
 *     "post_build" = 0,
 *   }

Every stage of processor should implement the interface related to that stage (preQueryProcessorInterface, PreBuildProcessorInterface).

Processors can implement other Interfaces as well, see #2596359: Widget config: facet ordering for more information.

In last night's weekly facet-hangout, we discussed this issue, this is the conclusion regarding the stages:

Stages:

1. pre_query preQuery(): uses the query_type and the facetSource implementation to make sure the alteration to the query was added before the query is executed in the backend?
   Example: DepedencyProcessor
2. post_query (postQuery() ?): uses the query results and can alter those results
   Example: ValueCallbackProcessor
3. build (build() ): uses the results and can do alterations before the renderable array is created.
   Example: WeightProcessor

This is now done and the followups are now unblocked.