feat(billing): handle limit aggregation for llm requests DEV-1031

[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 #Support Docs Updates, 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

Enables aggregation of automated QA billing limits and the inclusion of these limits in service usage data returned by the API.

💭 Notes

We still don't actually have an automated QA feature yet, so this code doesn't do much on its own. But when the feature is built and creates usage counters (see #6262), these will be compared against "llm_requests_limit" metadata in our Stripe plans and included in the calculation of limit balances.

👀 Preview steps

1. On a Stripe-enabled instance, create a new user on a community plan.
2. In django-admin add "llm_requests_limit" to the community plan product metadata
3. Create a project and, using the django shell, add an NLP usage counter for LLM requests on the new project.
4. Check the API response at api/v2/organizations/{org_id}/service_usage/ and see that the usage is correctly compared to the new plan limit number in the balances field.

[magicznyleszek]

@jamesrkiger the title shouldn't have square brackets :)

[Guitlle]

LGTM