Add a getImplementationName method to VectorizationProvider that returns the name of the selected implementation

[tflobbe]

Description

This draft is a possible solution for #15292. I implemented the getName() as the simple class name, but could also be just a string that represents the class, like default/panama. With this new method, the initialization of the Holder.INSTANCE could happen from a caller that's not in the VALID_CALLERS. It would not be returned though, I'm not sure if that's a problem. getInstance would still require valid callers. An alternative approach could be to refactor the lookup method in a way that the getImplementationName could use to determine the implementation without actually initializing it.