EmbeddedDocumentField errors during update when adding new field to DynamicEmbeddedDocument

Summary: When using a DynamicEmbeddedDocument, an InvalidQueryError occurs if you try to set a field that is not defined in the MongoEngine schema. This invalidates the purpose of a dynamic document.

Example:

class B(DynamicEmbeddedDocument):
   foo = StringField()

class A(Document):
  b = EmbeddedDocumentField(B)

# This works
A.objects(...).update(set__b__foo="123")

# This does not work
A.objects(...).update(set__b__bar="123")
# mongoengine.errors.InvalidQueryError: Cannot resolve field "bar"

Solution:
In my custom fork of MongoEngine, I've updated the EmbeddedDocumentField to account for dynamic documents as such.

class EmbeddedDocumentField(BaseField):
      def lookup_member(self, member_name):
        doc_and_subclasses = [self.document_type] + self.document_type.__subclasses__()
        for doc_type in doc_and_subclasses:
            field = doc_type._fields.get(member_name)
            if field:
                return field
        # NEW CODE
        if member_name not in ("S", "$") and any(issubclass(doc_type, DynamicEmbeddedDocument) for doc_type in doc_and_subclasses):
           return DynamicField(db_field=member_name)

Custom Situation: I encountered this error because I had a EmbeddedDocumentListField of DynamicEmbeddedDocuments and I was trying to perform array operators like so:

A.objects(b__foo="abc").modify(set__list_of_b__S__bar="123")

If the maintainers find my solution acceptable, I would be happy to write tests and make a PR with it. Otherwise, I will defer to coders with greater expertise in this codebase for the proper strategy here.

Start by reproducing the provided update examples and inspect the EmbeddedDocumentField.lookup_member implementation. Add regression tests for setting an undefined field on a DynamicEmbeddedDocument, including the EmbeddedDocumentListField array-operator case, and consider the S and $ path markers shown in the proposed change. Done means these updates no longer raise InvalidQueryError while normal field lookup remains intact.