feat: add parent_id column to workspace_agents table (#17758)

Adds a new nullable column `parent_id` to `workspace_agents` table. This
lays the groundwork for having child agents.

coderd/provisionerdserver/provisionerdserver.go

@@ -2085,6 +2085,7 @@ func InsertWorkspaceResource(ctx context.Context, db database.Store, jobID uuid.
 		agentID := uuid.New()
 		dbAgent, err := db.InsertWorkspaceAgent(ctx, database.InsertWorkspaceAgentParams{
 			ID:                       agentID,
+			ParentID:                 uuid.NullUUID{},
 			CreatedAt:                dbtime.Now(),
 			UpdatedAt:                dbtime.Now(),
 			ResourceID:               resource.ID,

coderd/provisionerdserver/provisionerdserver_test.go

@@ -2158,6 +2158,7 @@ func TestInsertWorkspaceResource(t *testing.T) {
 		require.NoError(t, err)
 		require.Len(t, agents, 1)
 		agent := agents[0]
+		require.Equal(t, uuid.NullUUID{}, agent.ParentID)
 		require.Equal(t, "amd64", agent.Architecture)
 		require.Equal(t, "linux", agent.OperatingSystem)
 		want, err := json.Marshal(map[string]string{