Support EditRouter in delete command

[NickWu]

Hi, in our pipeline, we have a tool in Maya to delete prims from a specified layer using USD's native API, and we are using the delete key as the hotkey to run the tool. In order the make the prim deletion behaviour consistent between our tool and in Maya viewport (so that users could hit the delete key either with the tool or viewport in focus and get the same result), we'd like to map the delete key in Maya to our prim deletion tool, but we learned that the delete key executed in viewport cannot be remapped and it has been reserved to Maya's cmds.delete() command which has its own logic to delete the selected prims from the layers retrieved based on its own logic. cmds.delete() calls UsdUndoDeleteCommand internally and it doesn't support EditRouter to get an edit target layer from users.

We made these changes in our pipeline to be able to send an UsdEditTarget to UsdUndoDeleteCommand via EditRouter so that a prim can be deleted in a layer we specify. By sending a UsdEditTarget object instead of a SdfLayer, clients have full control on how to construct a UsdEditTarget; if it's constructed with a pcp node, it can delete prims defined in non-local layers, which is one of the main intentions of this update.

The changes are:

• add getEditRouterEditTarget function in EditRouter to retrieve UsdEditTarget.
• turn on allowStronger in UsdUfe::applyCommandRestrictionNoThrow to pass the check.
• add new EditRouter tokens for registering new EditRouter callback for deletion and retrieving UsdEditTarget from callbacks.
• update wrapEditRouter to be able to retrieve UsdEditTarget object from Python.

[pierrebai-adsk]

Need to fix teh edit context and add a few comments in the code to help us underatdn what is going on in the future.

[NickWu]

Hi @pierrebai-adsk , I have added some comments as requested.

[pierrebai-adsk]

Ran internal preflight #1847 and it passed.