Plugin type to allow other ways to determine files to be indexed

bradley-b → changed the visibility of the branch 3510920-file-aggregator-plugin-type to hidden.

I think a plugin type for extracting files from a node is definitely the right way to go, as we have several issues where we need to expand how files are obtained:

- layout builder items - ✨ Make it possible to index attachments from index only fields that don't exist on the entity Active
- nested references such as paragraphs: ✨ Impossible to index attachment in nested entity reference Needs review

There's a couple of important changes I'd say the MR here needs though:

1. It should be possible to have multiple plugins active. For example, you might want plugins from layout builder AND from files referenced in body text tags.
2. Some plugins need to be recursive, and run enabled plugins on what they find. For example, you might want the paragraphs plugin to then run the body text plugin on the body fields on the referenced paragraph entities.