[FEATURE] Late Filtering Operator for Queries

[ppanopticon]

Description

In the XReco context (and potentially for some other applications) we require late-stage filtering for queries. This type of filter removes Retrievables from a result stream generated by some upstream Retriever.

## Solution
I'd propose to have a LateFilter that can do late-stage filtering of Retrievables based on some boolean predicate. The way this filter works is roughly as follows:

• It fetches the necessary field from the schema.
• It performs a comparison of the field's value with a predicate.
• It removes all Retrievables that don't match the predicate.
• As a side-effect, the LateFilterOperator can append the fetched field to the Retrievable (similar to the FieldLookup).

For the sake of simplicity, a first implementation compares a single field to a single predicate. Logical combinations (AND, OR) are not supported. There are some implementation details to consider and optimise here:

• Batch processing vs. stream processing for large result-sets
• Attribute matching for structs

## Dependencies
No know dependencies.

Context

It is currently not in scope to push-down these filters and combine them with the retrievable operation (which could be done, e.g., in PostgreSQL). This would constitute a next step.

[net-cscience-raphael]

See #123