core: Per-request caching for serialized users

[joaocfernandes]

Details

Improve execution time for calls to groups/.

For situations where a huge number of groups exists and a huge number of users scattered around them, we can save time avoiding re-serialization, if the serialization result for each user is cached by request.

This has the tradeoff of creating a cache per request. The invalidation model is simple as it only lives for the period of the request and therefore it's expected a low-memory footprint , if page_size is within a reasonable value.

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Checklist

• Local tests pass (ak test authentik/)
• The code has been formatted (make lint-fix)

If an API change has been made

• The API schema has been updated (make gen-build)

If changes to the frontend have been made

• The code has been formatted (make web)

If applicable

• The documentation has been updated
• The documentation has been formatted (make docs)

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