Make service for field discovery for use in migrate entity derivers

Problem/Motivation

In contrib (and even in core itself), there is a lot of boilerplate repeated when a migrate deriver wants to add fields to an source migrate deriver. Let's make the code for discovering and adding fields generic and make a re-usable trait.

Paragraphs and Commerce would use this. Node, user and taxonomy term in core could also.

Proposed resolution

Take a look at node source deriver, make a trait and then implement it for node, user and taxonomy.

Remaining tasks

1. Code it
2. Run existing tests
3. Commit it

User interface changes

API changes

Data model changes