rest_framework_gis support doesn't work if source points to a relationship

[AndrewGuenther]

Describe the bug
If I put a GeometryField on a serializer that has a source like geo.feature, the rest_framework_gis extension can't walk that relationship and defaults to string type.

To Reproduce
Here's a code example:

class AssetSerializer(serializers.ModelSerializer):
    aoi = GeometryField(source="aoi.geometry")

Expected behavior
I would expect that the relationship specified in source would be walked properly to identify the field type of geometry

[AndrewGuenther]

https://github.com/tfranzel/drf-spectacular/blob/master/drf_spectacular/contrib/rest_framework_gis.py#L213

This line is the culprit. It does a naive field lookup which can't walk relationships.

[tfranzel]

Hey @AndrewGuenther, good catch. could you try out #815 if that solves your issue?

[AndrewGuenther]

I'm going to be on other projects through the end of this week, so I'll circle back and test this on Monday.