chore(all): return concrete types, accept interfaces

- Remove exported interfaces unused locally
- Define interfaces to accept arguments
- Return concrete types, not interfaces

internal/publicip/state/publicip.go

@@ -4,11 +4,6 @@ import (
 	"github.com/qdm12/gluetun/internal/publicip/models"
 )
 
-type DataGetSetter interface {
-	GetData() (data models.IPInfoData)
-	SetData(data models.IPInfoData)
-}
-
 func (s *State) GetData() (data models.IPInfoData) {
 	s.ipDataMu.RLock()
 	defer s.ipDataMu.RUnlock()