Provide a replacement for hook_page_manager_variant_operations_alter()

It needs to be possible to build extra operations, similar to hook_page_manager_variant_operations_alter() in D7. This would be useful for e.g. adding a custom tab/subpage for adding meta tags via the Metatag module (#2563657: Panels / Page Manager integration), etc.

Per @eclipsegc:

ok, so practically, I think the API you're asking for is missing. That being said, while it is DEFINITELY suboptimal, I think you can probably look at page_manager_ui_entity_type_build() and emulate what's happening there. I'd build a subclass of which ever wizard makes the most sense (probably variant) and add my step manually through that process.