Replace EntityEncoderFilter with per-entity object filter

[andrus]

As mentioned in #445 , EntityEncoderFilter is not that easy to use:

• Mixes encoding and access control logic
• Requires logic duplication between willEncode and encode
• Is not strictly per-entity (though there are per-entity utilities to create filters)

Let's design a new ReadFilter API that will allow to filter objects in GET or POST/PUT requests before sending them to client based on custom logic. It should be per-entity, and internally attached to AgEntity/AgEntityOverlay, just like property filtering is. It should also address a recently deprecated use case of Constraint.qualifier.

Notes on the implementation:

• EntityEncoderFilter is deprecated in 4.8 and will be removed in 5.0.
• A new SelectStage.FILTER stage is added at the end of the select pipeline
• Here is how the new ReadFilter API works:

// per-request
Ag.service(config)
    .select(E.class)
    .filter(E.class, e -> /* eval some condition */))
    .filter(A.class, a -> /* eval some condition */))
    .get();

// per-runtime
new AgBuilder()
    .entityOverlay(
        AgEntity.overlay(E.class).readFilter(e -> /* eval some condition */))
    )
   .entityOverlay(
        AgEntity.overlay(A.class).readFilter(a -> /* eval some condition */))
    )
    .build();