Add read_usage_quotas ability to GroupPolicy

The following discussion from !82396 (merged) should be addressed:

• @toupeira started a discussion: (+2 comments)

  thought (non-blocking): I saw we have a similar situation with :admin_group in https://gitlab.com/gitlab-org/gitlab/blob/e7983f2e29073250550fc6df3fdb2e7973863d66/ee/app/controllers/groups/usage_quotas_controller.rb#L4, it probably makes sense to add a :read_usage_quotas on GroupPolicy as well.

  This could definitely be its own follow-up MR though 😀

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Proposal

1. add a new ability, read_usage_quotas, to the GroupPolicy
2. modify the Groups::UsageQuotasController before action to use the new ability
3. add test coverage

Availability & Testing

Unit test coverage should be sufficient