Supporting maxmind country db in geoip maxmind provider

*Title*: *Supporting maxmind country db in geoip maxmind provider*

*Description*:
Currently the geoip maxmind provider supports city, isp, and anonymous database files. In cases where only country-level information is needed, it's desirable to have the option of loading only the country database, because the country database is a lot smaller (5-6MB) than city database (~100MB) and thus can be more efficient.

[optional *Relevant Links*:]
maxmind provider config api: https://www.envoyproxy.io/docs/envoy/latest/api-v3/extensions/geoip_providers/maxmind/v3/maxmind.proto#envoy-v3-api-msg-extensions-geoip-providers-maxmind-v3-maxmindconfig