MAYA-128513 implement code wrapper handler

[pierrebai-adsk]

Implement a code wrapper handler to support edit routing for composite commands:

• Make the batch-ops handler derive from the correct class depending on which feature is supported in UFE.
• Implement a code wrapper handler derived from the batch ops handler.
• This code wrapper handler creates edit-routing wrappers for composite commands.

Implement a code wrapper that does the edit routing:

• The code wrapper cache the stage and layer used when first executing a composite command.
• This allows using the same values when undoing.
• This is necessary because the original prim that was used when executing the command might not exist anymore once the command executed.
• In particular, this is true for the group command, the original prim no longer exists.

Make the group command work with edit routing. Note that the Maya group command is implemented in term of the insert-child UFE command. That is why we are modifying the insert-child command.

• Use the generic undo/redo support to record the changes, undo and redo them.
• Don't assume that the current edit target is where the source prim has opinion.
• Instead, apply the child-insertion to the layer that have opinion about the moving prim.
• The command restrictions already ensure that we only affect prims that have a single opinion.
• (But with these changes, the code will be ready if we support multi-layers command later on.)
• The problem was that the command restrictions were correctly looking at where the prim was authored, but the command execution was assuming that the source opinion was on the target layer.
• When routing commands, that can obviously be false: the target might not be where the original opinions were.
• We do the right thing using similar code as when affecting the session layer, except the target layer is the layer where the command was routed.
• By default, when no routing is registered, that is the current edit target.
• Fix load rules duplication: the Load rules are not part of the USD data that gets restore by automatic USD undo block, so we need to duplicate them explicitly. Also, the UFE reparent notification need to be triggered during undo and redo.

Fix unit tests:

• The unit tests were tied to some subtle behavior of the implementation. Now that we use "perfect" undo/redo, the order of items is also preserved, so the testGroupCmd assertions had to be fixed. Similarly, the parent command was assuming the parent prim was passed to the edit router. It was also creating the parent prims, which is not necessary. (The problem was fixed before, but the fix was hidden due to the exact timing of the edit routing call.)
• Also made the parent unit test for edit routing properly unregister the router even when the test fails. Otherwise, the subsequent test crashes due to the invalid edit router being left behind.
• Added a unit test for the Maya group composite command.
• Made sure the code wrapper unit test is not run when the API is not available.
• Made text filtering ignore all white spaces at the start and end of lines to avoid to have to write the exact indentations.

[seando-adsk]

Overall the changes look good. Great work. Just a few minor things to fix.