feat(billing): switch asset usage endpoint to limit offset pagination DEV-987

[jamesrkiger]

🗒️ Checklist

1. run linter locally
2. update developer docs (API, README, inline, etc.), if any
3. for user-facing doc changes create a Zulip thread at #Kobo support docs, if any
4. draft PR with a title <type>(<scope>)<!>: <title> DEV-1234
5. assign yourself, tag PR: at least Front end and/or Back end or workflow
6. fill in the template below and delete template comments
7. review thyself: read the diff and repro the preview as written
8. open PR & confirm that CI passes & request reviewers, if needed
9. delete this section before merging

📣 Summary

Removes custom asset usage page-size pagination, thereby using KPI's default limit-offset pagination, and adjusts drf_spectacular code for the endpoint to improve API docs/frontend helpers.

💭 Notes

I am marking this as a draft for the time being, but it is ready for review. We will merge it as part of the corresponding frontend changes (See DEV-1013).

👀 Preview steps

No need for testing on this. Testing will come with frontend PR.

[rgraber]

LGTM

[jamesrkiger]

This is the main difference from how I've seen the drf_spectacular code used in other parts of the app. I thought it would be preferable to use the actual AssetUsageSerializer in views.py, rather than creating an inline serializer. To me, it makes the code more readable. But I can also see that there might be some confusion of concerns, since the NlpUsageSerializer here is being passed into @extend_schema_field rather than used directly in the AssetUsageSerializer. Hoping to get your input on this, @noliveleger.