The log in `NewFrameItemFromRegionKey` is not clear when the key is not utf8 string

```go
logutil.BgLogger().Warn("decode region key failed", zap.ByteString("key", key), zap.Error(err))
```

In `NewFrameItemFromRegionKey`, we used `zap.ByteString` to print the key. However, if the key is not a valid utf8 string, it'll be turned into a truncated utf8 string with a lot of `\uFFFD`. It's not helpful for debug.