Add a Go package for reading MaxMind GeoLite2 City and ASN databases.
- ParseConf: parses geoipupdate-style config files (AccountID,
LicenseKey, EditionIDs, DatabaseDirectory)
- Databases: holds open City + ASN readers; Open() loads both
editions from a directory, extracting .mmdb from tar.gz in memory
without writing to disk
- FindTarGz: resolves cache paths, preferring *_LATEST.tar.gz
- Lookup: returns city, region, country, ASN, and AS org for an IP
- DownloadBase constant, TarGzName helper, DefaultConfPaths and
DefaultCacheDir conveniences
Integration tests verify download and conditional-get freshness.
