Implement license enforcement for AI agents

[deansheather]

Part of phase 0 of the Usage-based billing RFC.

https://www.notion.so/coderhq/Usage-based-Billing-AI-b-210d579be592800eb257de7eecd2d26d

• Adds two new license features managed_agent_limit_soft and managed_agent_limit_hard
  • These two license features get merged into a single entitlement feature managed_agent_limit
• Usage gets calculated over the license window