Add betatesters listbygroup command

[n-a-f-i-s-a]

closes #176

📝 Summary of Changes

Changes proposed in this pull request:

• Add beta testers listbygroup command to get beta testers for a specific app and specific group name

🔨 How To Test

swift run asc testflight betatesters listbygroup iba.test3 testGrp1 swift run asc testflight betatesters listbygroup 1511865740 testGrp1 swift run asc testflight betatesters listbygroup iba.test2 testGrp2