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/noise.py

@@ -5,13 +5,13 @@ import torch
 from pydantic import field_validator
 
 from invokeai.app.invocations.latent import LatentsField
+from invokeai.app.shared.fields import FieldDescriptions
 from invokeai.app.util.misc import SEED_MAX, get_random_seed
 
 from ...backend.util.devices import choose_torch_device, torch_dtype
 from .baseinvocation import (
     BaseInvocation,
     BaseInvocationOutput,
-    FieldDescriptions,
     InputField,
     InvocationContext,
     OutputField,