feat: add group configuration (+)

will only check users of such group

also cleaned some code:
- Input doesn't need to be UTF-8 valid.
- Main fuction is dead-simple, very readable.
- Added some modularity.
- Add some docs on behavior details.

Cargo.toml

@@ -17,6 +17,7 @@ trim = true
 
 [dependencies]
 curl = { version = "0.4.49", default-features = false }
+libc = { version = "0.2.186", default-features = false }
 nonstick = { version = "0.1.2", default-features = false, features = ["link"] }
 serde = { version = "1.0.228", default-features = false, features = ["derive"] }
 serde_json = { version = "1.0.149", default-features = false, features = [