Network Feature Config endpoint: filter name for network_service_config

Naoya from JPNAP raised awareness about an inconsistency in the current spec.

https://docs.ix-api.net/v2/#operation/network_feature_configs_list

Query parameter service_config should be network_service_config

We suggest:

• mark the current filter as deprecated
• introduce network_service_config filter to avoid a BREAKING CHANGE
• remove the filter in v3