Add support for mapping all nullable types as pointers (#198)

This implements the approach suggested in
https://github.com/Khan/genqlient/issues/178#issuecomment-1077559343.
See the added documentation for the full behavior.

docs/genqlient.yaml

@@ -98,6 +98,23 @@ use_struct_references: boolean
 use_extensions: boolean
 
 
+# Customize how optional fields are handled.
+optional:
+  # Customize how models are generated for optional fields. This can currently
+  # be set to one of the following values:
+  # - value (default): optional fields are generated as values, the same as
+  #   non-optional fields. E.g. fields with GraphQL types `String` or `String!`
+  #   will both map to the Go type `string`. When values are absent in
+  #   responses the zero value will be used.
+  # - pointer: optional fields are generated as pointers. E.g. fields with
+  #   GraphQL type `String` will map to the Go type `*string`. When values are
+  #   absent in responses `nil` will be used. Optional list fields do not use
+  #   pointers-to-slices, so the GraphQL type `[String]` will map to the Go
+  #   type `[]*string`, not `*[]*string`; GraphQL null and empty list simply
+  #   map to Go nil- and empty-slice.
+  output: value
+
+
 # A map from GraphQL type name to Go fully-qualified type name to override
 # the Go type genqlient will use for this GraphQL type.
 #