add cache interface

cache/cache.go

@@ -0,0 +1,13 @@
+package cache
+
+import (
+	"context"
+	"time"
+)
+
+// Cache is a generic key value cache interface.
+type Cache interface {
+	Get(ctx context.Context, key string) (interface{}, error)
+	Put(ctx context.Context, key string, value interface{}, expired time.Duration) error
+	Delete(ctx context.Context, key string) error
+}