Add an attribute parsing cache collector discovery for plugins

Problem/Motivation

Follow-up from #3486503: Add a file parsing cache collector to replace some uses of FileCache.

Attribute parsing has to load the PHP class into memory which can then never be freed. If we apply the same approach from that issue we should almost never have to load all the classes at once, only when they change.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet