Describe the bug
PartitionField and SortField don't conform to SPEC because property names are using snake case, instead of kebab case.

To Reproduce

PartitionField:

1. field_id -> field-id ref
2. source_id -> source-id ref

SortField:

1. source_id -> source-id ref
2. null_order -> null-order ref

Expected behavior
Types use kebab case.