Add an EntityFinder service

Problem/Motivation

In #3073275: Mapping to books I have been working on a new service called "feeds.entity_finder". Its implementation duplicates some code found in the FeedsTarget plugins "entity_reference" and "config_entity_reference". There's also no test coverage for that new service in that issue, hence why the code for that issue hasn't been committed yet.

So let's create a spin-off issue, by implementing the Entity finder service separately from the book target. And let the entity reference targets make use of this new service.

Proposed resolution

Create a new service called "feeds.entity_finder". Refactor the FeedsTarget plugins "entity_reference", "file", "config_entity_reference" and "user_role" to make use of this new service.

Remaining tasks

1. Add the service "feeds.entity_finder".
2. Refactor FeedsTarget plugins.
3. Update the unit tests for these targets.
4. Update the kernel tests for these targets.
5. Review

User interface changes

None.

API changes

The implementation of some FeedsTarget plugins will change. This could have effect on plugins that extend these FeedsTarget plugins.

Data model changes

None.