USD "save as" after a Stage created from file

**Is your feature request related to a problem? Please describe.**
Currently, once create a stage from file, we can not save the same stage as a different path or change file format. It would be nice to have an option to change path or file format of the same stage.

**Describe the solution you'd like**
in USD Layer Editor, a button to change path o "save as", not just "save edits" as it is now.

**Describe alternatives you've considered**
main features are the possibility to create copy of that stage, change file format or the main path with of file

thank you

Start in the USD Layer Editor and reproduce the reported workflow: create a stage from a file, then use the existing save-edits behavior. Determine how a copy, a new path, and a changed file format should work; the feature is done when the editor provides a save-as option for the same stage.

Written by the indexing model from the issue text.