feat: add managed agent license limit checks (#18937)

- Adds a query for counting managed agent workspace builds between two
timestamps
- The "Actual" field in the feature entitlement for managed agents is
now populated with the value read from the database
- The wsbuilder package now validates AI agent usage against the limit
when a license is installed

Closes coder/internal#777

coderd/workspaces.go

@@ -701,7 +701,7 @@ func createWorkspace(
 			return xerrors.Errorf("get workspace by ID: %w", err)
 		}
 
-		builder := wsbuilder.New(workspace, database.WorkspaceTransitionStart).
+		builder := wsbuilder.New(workspace, database.WorkspaceTransitionStart, *api.BuildUsageChecker.Load()).
 			Reason(database.BuildReasonInitiator).
 			Initiator(initiatorID).
 			ActiveVersion().