Follow-ups from "Add GraphQL trial usage query for SaaS and self-managed"

Context

A few suggestions were identified in the review of https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/14661 but none were deemed blockers for that MR. These would be nice to include as a follow-up though.

@tyleramos started a discussion: (+3 comments)

Question (non-blocking): For subscriptions, the equivalent class is Resolvers::Subscriptions::GitlabCredits::UsageResolver. Do you think it would be useful to nest this the same way with GitlabCredits (e.g. Resolvers::Trials::GitlabCredits::UsageResolver)?
@vburton started a discussion: (+1 comment)

Suggestion (non-blocking): Not a blocker, more of a possible future improvement - it might be good to look into reusing the same Billing::Usage::TrialsFinder, but add a for_namespace method similar to what Billing::Usage::SubscriptionsAndTrialsFinder defines. That way we're using the same underlying query for trial lookups that the consumer resolver and enrichment use as well.
@vburton started a discussion:
Suggestion, non-blocking: Can also be addressed in a follow-up - for better accuracy, it would be good to add the :trial and :cloud traits here and for the rest of the create(:license) calls in this spec:
```
    let(:license) { create(:license, :cloud, :trial, zuora_subscription_name: trial.subscription_name) }
```