Implement usage APIs

[easyCZ]

Depends on #10324.

With the API definition in place, implement the API. Recommended approach:

• First define a stub API which returns mocked out data
• Second, implement necessary access controls and restrictions (including feature flag check)
• Third, implement actual logic for data retrieval.
• Add a "time range filter" parameter

[geropl]

Dropping work here, as the "wiring" between dashboard, server and usage is done. Leaving for @andrew-farries to pick it up. 💪