Choose valid id if id is provided for content creation

By now content creation, or concrete adding created content to its container, fails, if content with this id provided by the api call already exists in its container.

Steps to reprouce:
POST request to /mypage with id "foo", etc
Another POST request to /mypage with id "foo", etc

The reason, the use case, for this is:
In Volto you can create content, that gets an id that clashes with for example routes that only Volto knows, not the backend. Create a page with title "Register". You are redirected after creation to the register form.
So if in Volto the id is explicitly provided, like for example by appending "-1": "register-1", on REST api call, this would solve the problem, nearly. Do it twice, and it fails, cause plone.restapi does not apply INameChooser(container).chooseName for a call WITH id.

Reproduce the issue with two POST requests to /mypage using the same explicit id, such as "foo". Trace the content-creation path that handles an explicitly provided id and compare it with the naming behavior for generated ids. Done means repeated creation requests choose a valid non-conflicting id instead of failing.