Forward a destination query parameter on entity add pages

This issue is being reviewed by the kind folks in Slack, #needs-review-queue-initiative. We are working to keep the size of Needs Review queue [2700+ issues] to around 400 (1 month or less), following Review a patch or merge request → as a guide.

This will need test coverage.
Left a comment but think we should make the new parameter default to NULL for backwards compatibility. With a trigger_error it will be required in D11.

Please make this more generic and pass through all query parameters. While destination needs special handling, other query parameters could just be passed through. Or if that is too open maybe trigger an event to retrieve the query parameters to pass for a specific request.

In my use case I need a query parameter "owner" in order to set the owner of the new entity to a defined user passed via query parameter (and hiding the author selection UI) instead of the current user creating the new entity. The entity is a commerce product which should be owned by the "seller" user (which has an "add product" action on its profile, accessible to the seller user and all shop managers + admins) and not the "shop manager" or admin user creating the product. Without this issue being generic my only option would be to add separate local actions per product bundle.

Re #4: Thanks for the review! Added a draft change notice and the in-code deprecation. The merge request already adds a bunch of assertions to EntityAddUITest so what test coverage are you looking for exactly?

Re #5: I understand the use-case totally, but given that it's already hard enough to find support for this very targeted issue I don't believe broadening the scope is wise at this point. Feel free to post a separate MR (here or in another issue) for your use-case but I will continue with the current approach for now, sorry.