Expose `add` method on registry object about protobuf-es

[timostamm]

@jafaircl, #899 adds the function createMutableRegistry(), which creates a registry with a method add().

#898 adds a similar function to the upcoming version 2.

[timostamm]

Hey @jafaircl, today, registries are very simple interfaces that do not allow mutation. And it wouldn't be trivial to add an add method to the interface because of cases like createRegistryFromDescriptors().

In the long term, we'd love to extend the capabilities (also see #708). For now, the best approach is to implement the interface yourself, and provide an add method.