feat: add getTypeUrl method to generated code

[taylorcode]

An encoded proto containing a nested Any that is missing a typeUrl cannot be decoded in Python, and perhaps other languages.

js

const anyProto = google.protobuf.Any.create({
   // typeUrl: none provided
   value: arrayBuffer,
});

python

child_message = ChildMessage()
any_msg.Unpack(child_message) # fails because of missing type url on any_msg

It looks like in other languages, e.g. Java a nested Any message has a getTypeUrl method. It would be nice if protobufjs's Any also provided a similar method or property that would return the default type url.

Any childMessage = parentMessage.getChildMessage();
String typeUrl = childMessage.getTypeUrl()

It seems like in order to add this to Any, we would need messages to know their default typeUrl. In other languages this can be determined through reflection, but for JS this doesn't work reliable for at least two reasons. First, we'd need a way to determine the constructor name. instance.constructor.name gives us this, but it does not work in IE, and the workarounds seem wonky. Second, minifiers can change the name of constructors. In practice this usually shouldn't be a problem because protobufjs messages are properties on an object and not local variables (and therefore not mangle-able), but there are minifiers that can mangle properties, such as closure. Therefore I think the default type url needs to be represented as a string.

[alexander-fenster]

@taylorcode Thank you! We can merge it but I can't bring this PR up to date. Can you please press the Update button for me? Thank you!

[alexander-fenster]

@taylorcode Also, adding a test would've been appreciated - just to make sure this feature does not get broken by a random change in the future.

[taylorcode]

@alexander-fenster I've added a test but I'm not confident I did it correctly. I ran node scripts/gentests.js to regenerate the fixtures, but I don't see any existing tests on the generated code.

I noticed the test files within the tests directory follow a certain naming convention, test files on the protobuf library itself are prefixed with api_, tests on the proto parser are prefixed with comp_, docs with docs_, and some other tests with other_. I prefixed my test file with gen_.

[alexander-fenster]

@taylorcode Mind giving the reviewers a permission to update your branch? Thank you!

[taylorcode]

@alexander-fenster sorry, I'm not sure how to grant write access. Could you provide guidance? thanks