Add support for MaxMind GeoIP2 Enterprise and Anonymous-IP databases

[edmocosta]

• Added support for MaxMind GeoIP2 Enterprise and Anonymous-IP databases.
• Changed the database type detection to use the same strategy as com.maxmind.geoip2.DatabaseReader#getDatabaseType, so it doesn't rely on specific database types.
• Updated MaxMind dependencies.
• Added unit tests for the Java classes, including the verification of the plugin's basic operations against all supported databases.
• The gem version is now read from the VERSION file.

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Closes: https://github.com/logstash-plugins/logstash-filter-geoip/issues/153 Closes: https://github.com/logstash-plugins/logstash-filter-geoip/issues/212