Allow to alter revision candidates [#3216266]

Problem/Motivation

It would be great to be able to alter the getCandidatesRevisions query, along the same lines as what was done for the getCandidatesNodes query in https://www.drupal.org/project/node_revision_delete/issues/2905912.

This could be helpful in complex workflows with different "published" states.
For example, a workflow might have a "pre-published" moderation state whose revisions should never be deleted.

Proposed resolution

Add the following tags to the sub-query:
- 'node_revision_delete_candidate_revisions'
- 'node_revision_delete_candidate_revisions_' . $content_type

Remaining tasks

- ~~add tags~~
- ~~write tests~~

Issue fork node_revision_delete-3216266

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3216266-allow-alter-revision-candidates changes, plain diff MR !2
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