Update set what-if handling

[tgauth]

PR Summary

• adds whatIfArg within set block in manifest
• adds deprecation warning when what-if block in manifest is used and removes what-if from capabilities for dsc list
• splits process_args() into process_get_args() and process_set_delete_args()

PR Context

• fix Add new arg type for set and delete to support what-if #1361 (delete PR to follow)