Create usage-based billing package

[deansheather]

Part of phase 1 of Usage-based Billing RFC

We will add a new package called usage which contains all of the storage and publishing code for usage events.

• Enterprise licensed code, using a collector interface defined in AGPL + a no-op stub

  type EventType string // enum
  type UsageEvent interface {
    usageEvent() // to enforce they must come from this package
    EventType() EventType
  }
  type UsageCollector interface {
    RecordUsage(ctx context.Context, event Event) error
  }

• Adds a new usage_events table as per the RFC
• All of the event structs must be versioned
• CODEOWNERS should include @deansheather to prevent people from changing the structs unintentionally
• Publishes to Tallyman with a goroutine as outlined in the RFC
  • Only if the deployment has any license with publish_usage_data=true
• Generate an initial dc_ai_workspaces_v1_genesis event with existing data ONCE
• Add code to the provisionerd server to record an event when new has_ai_task workspaces are built
• Change the managed agent count function to count events from the new table rather than from the workspace_builds table