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/workspacebuilds.go

@@ -335,7 +335,7 @@ func (api *API) postWorkspaceBuilds(rw http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	builder := wsbuilder.New(workspace, database.WorkspaceTransition(createBuild.Transition)).
+	builder := wsbuilder.New(workspace, database.WorkspaceTransition(createBuild.Transition), *api.BuildUsageChecker.Load()).
 		Initiator(apiKey.UserID).
 		RichParameterValues(createBuild.RichParameterValues).
 		LogLevel(string(createBuild.LogLevel)).