Prim metadata editRouting

[jufrantz]

Following #3778, this proposal introduces support for a new registrable primMetadata router, designed similarly to attribute routing. A unique router will receive the USD metadata name in its context. For customData and variantSelection, the router will also receive a dictionary key path, such as the variantSet name for variantSelection.

These commands support this routing:

• UsdUndoToggleInstanceableCommand
• UsdUndoToggleActiveCommand
• UsdUndoSetKindCommand
• UsdUndoAddReferenceCommand
• UsdUndoAddPayloadCommand
• UsdUndoClearPayloadsCommand
• UsdUndoClearReferencesCommand
• SetVariantSelectionCommand
• ClearSceneItemMetadataCommand
• SetSceneItemMetadataCommand

I still need to add unit tests and update EditRouting.md. But before going further, I wanted to push these initial changes to ensure they fit the project.