Specify custom json serializers

For example, if I am using mongodb, I currently have to serialize the responses I get myself using `bson.json_util.dumps`. It would be nice if we could specify the default json serializer that chalice uses.

The issue names no repository files or tests; start by tracing where Chalice serializes API responses and compare the current behavior with bson.json_util.dumps. Done means users can configure a default JSON serializer and responses use it without manual serialization.

Written by the indexing model from the issue text.