Improve DX for implementing custom entity URL generators.

Problem/Motivation

The generator plugins follow a standard pattern:
1. Generate meta-data.
2. Filter link entries based on the aggregated meta-data.
3. Return the data structure needed for the site-map. (this should be moved to shared base class

The DX problem I am noticing is that Whenever I want to reuse a particular base class, I need to copy all 3 of the steps to implement anything custom in there.

This is causing:
- Duplicated effort
- Maintenance overhead.

For a concrete example, let's look into EntityUrlGenerator::processDataSet($data_set).
It is doing filtering of links, based on different conditions, every time it returns FALSE.
If you want to add a condition to filter the result, you will have to re-implement the whole method again just for the 2 lines needed.

There are cases (pretty much every time) in decoupled Drupal sites where Drupal will generate sitemaps for different domains than it currently runs on. Again just to change that 2 lines of code, the whole method needs to be re-implemented.

Proposed resolution

Introduce this designated steps as methods and sub-methods that will perform one thing only instead of all of them in a single processDataSet($data_set).

Have a utility data array that will aggregate all the meta-data used for filtering.
Have all the filter conditions moved to a separate function and aggregate that meta-data array in there.

Have the URL generation moved itself to a utility method that will allow overwriting as well.

Remaining tasks

Discussion, PoC patch etc.

User interface changes

None.

API changes

New methods to extend for anyone that needs to implement custom URL providers.
Should be backwards compatible change.
Anyone that has currently customized that method will have done so by rewriting the whole thing.

Data model changes

None.

Release notes snippet

Refactoring of internal protected APIs.