epic: Usage-based billing (Managed Agent Usage) RFC

[Usage-based Billing RFC](https://www.notion.so/coderhq/Usage-based-Billing-AI-b-210d579be592800eb257de7eecd2d26d)

Phase 0:
- https://github.com/coder/internal/issues/760
- https://github.com/coder/internal/issues/777
- https://github.com/coder/nexus/issues/245
- https://github.com/coder/internal/issues/813

Phase 1:
- https://github.com/coder/internal/issues/814
- https://github.com/coder/tallyman/issues/1
- https://github.com/coder/tallyman/issues/2

Note: between phase 1 and phase 2 will likely be a lot of non-product work required to get Metronome up and running before we give customers licenses.

Phase 2:
- https://github.com/coder/tallyman/issues/3
- https://github.com/coder/internal/issues/817

Phase 3:
- https://github.com/coder/tallyman/issues/4

Phase 4:
- TBD