Support Media Message (#9)

internal/wechatdb/datasource/datasource.go

@@ -30,6 +30,9 @@ type DataSource interface {
 	// 最近会话
 	GetSessions(ctx context.Context, key string, limit, offset int) ([]*model.Session, error)
 
+	// 媒体
+	GetMedia(ctx context.Context, _type string, key string) (*model.Media, error)
+
 	Close() error
 }