java_gen.OFMatchV3V13: auto-gen list of supported fields

Reviewer: @vaterlaus @ronaldchi @meichi

This pull request makes the list of supported matchfields in OFMatchV3Ver13 be autogenerated from the java_model's OXM Map. This should help us avoid situations where we forget to add a constant to the method (e.g., BSN_LAG_ID is missing below).