chore(all): return concrete types, accept interfaces

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

internal/provider/custom/interfaces.go

@@ -0,0 +1,8 @@
+package custom
+
+import "github.com/qdm12/gluetun/internal/models"
+
+type extractor interface {
+	Data(filepath string) (lines []string,
+		connection models.Connection, err error)
+}