Immutable implementations of INodeTypeProvider

https://github.com/aspnet/EntityFrameworkCore/issues/12682#issuecomment-407530293

The implementations of INodeTypeProvider should offer an immutable implentation for use after instantiation. This can be realized by adding an AsReadOnly() method to the concrete implementations or making this an explicit builder pattern. If this can be designed as an additive feature / nonbreaking change, the implementation could be done in v2.3.