chore(all): return concrete types, accept interfaces

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

internal/loopstate/get.go

@@ -2,10 +2,6 @@ package loopstate
 
 import "github.com/qdm12/gluetun/internal/models"
 
-type Getter interface {
-	GetStatus() (status models.LoopStatus)
-}
-
 // GetStatus gets the status thread safely.
 func (s *State) GetStatus() (status models.LoopStatus) {
 	s.statusMu.RLock()