Add POST /produce endpoint

internal/http/interfaces.go

@@ -0,0 +1,10 @@
+package http
+
+import "github.com/davidlick/supermarket-api/internal/produce"
+
+type ProduceService interface {
+	Add(items []produce.Item) error
+	Remove(item produce.Item) error
+	Get(produceCode string) (item produce.Item, err error)
+	All() (items []produce.Item, err error)
+}