fix(nodes): create app/shared/ module to prevent circular imports

We have a number of shared classes, objects, and functions that are used in multiple places. This causes circular import issues.

This commit creates a new `app/shared/` module to hold these shared classes, objects, and functions.

Initially, only `FreeUConfig` and `FieldDescriptions` are moved here. This resolves a circular import issue with custom nodes.

Other shared classes, objects, and functions will be moved here in future commits.

invokeai/app/invocations/primitives.py

@@ -5,10 +5,11 @@ from typing import Optional, Tuple
 import torch
 from pydantic import BaseModel, Field
 
+from invokeai.app.shared.fields import FieldDescriptions
+
 from .baseinvocation import (
     BaseInvocation,
     BaseInvocationOutput,
-    FieldDescriptions,
     Input,
     InputField,
     InvocationContext,