Maint: rework publicip package

- Use loopstate package
- Loop interface composition
- Return concrete struct from constructors
- Split into more files
- Add publicip/state package

internal/publicip/alias.go

@@ -0,0 +1,6 @@
+package publicip
+
+import "github.com/qdm12/gluetun/internal/publicip/state"
+
+type Getter = state.PublicIPGetter
+type SettingsGetterSetter = state.SettingsGetterSetter