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/settings.go

@@ -0,0 +1,16 @@
+package publicip
+
+import (
+	"context"
+
+	"github.com/qdm12/gluetun/internal/configuration"
+)
+
+func (l *Loop) GetSettings() (settings configuration.PublicIP) {
+	return l.state.GetSettings()
+}
+
+func (l *Loop) SetSettings(ctx context.Context, settings configuration.PublicIP) (
+	outcome string) {
+	return l.state.SetSettings(ctx, settings)
+}