Allow restricting the fields fetched when using LazyReferenceField

Suppose we have the following document structure.

class Bar(Document):
    a_simple_field = StringField()
    a_huge_field = BinaryField()

class Foo(Document):
    some_info = StringField()
    bar = LazyReferenceField('Bar')

I would like to be able to call fetch() on the bar field of Foo without fetching a_huge_field from Bar. Maybe we can do something like:

a_bar = a_foo.bar.fetch(fields={'a_simple_field': 1})
another_bar = another_foo.bar.fetch(fields={'a_huge_field': 0})

Which follows the fields pattern: https://github.com/MongoEngine/mongoengine/blob/4caa58b9ec3afc856a7c05a2cdca9152b2c29e5f/mongoengine/queryset/base.py#L950.

I've opened a pull request with a possible implementation.

Start at the LazyReferenceField.fetch entry point and compare the requested fields pattern in queryset/base.py around line 950. Review pull request #2037 for the existing implementation and its coverage; done means the proposed fetch(fields=...) behavior is implemented and verified without retrieving excluded fields.