Add Button to Save All USD Layers as a Different Type

**Is your feature request related to a problem? Please describe.**
There seems to be no way to save a USD file in the USD Layer Editor as a different type without making a change. Since there's no way to directly edit USD files in Maya, I want to be able to save as ASCII to perform those changes with a simple text editor. Then I want to re-save all the layers as Binary for performance. Sometimes when making a change so I can undo it to re-save the file as an ASCII file will still write an empty "over" statement that impacts assets in scope.

**Describe the solution you'd like**
I'd like a re-save all as type button in the USD Layer Editor Option drop down menu.

**Describe alternatives you've considered**
I've considered writing a script that would make an edit and remove it from all the layers and then save, but that's definitely not optimal.

Start in the USD Layer Editor and inspect the Option drop-down menu and its existing save actions. Confirm how a save-as-type operation should apply to all layers, including ASCII and Binary output, without requiring an edit; verify that saving without edits does not add an empty "over" statement.

Written by the indexing model from the issue text.