import: JSON input processing supports --fields and --fields-from-header flags

[max-ipinfo]

These flags were only supported for CSV and TSV inputs.

Testing

Using https://github.com/ipinfo/sample-database/blob/main/IP%20Geolocation/ip_geolocation_sample.json:

$ mmdbctl import --json --fields city,region,country --in ip_geolocation_sample.json --out ./test.mmdb ; mmdbctl export test.mmdb | head -n2
writing to ./test.mmdb (100 entries)
range,city,country,network,region
14.3.247.44/31,Isesaki,JP,14.3.247.44-14.3.247.45,Gunma

$ mmdbctl import --json --in ip_geolocation_sample.json --out ./test.mmdb ; mmdbctl export test.mmdb | head -n2
writing to ./test.mmdb (100 entries)
range,city,country,latitude,longitude,network,postal_code,region,timezone
14.3.247.44/31,Isesaki,JP,36.31667,139.2,14.3.247.44-14.3.247.45,321-4342,Gunma,Asia/Tokyo