[Avro] Rename `VisitorFormatWrapperImpl` to `AvroFormatVisitorWrapper` in 2.14 or 3.0

`VisitorFormatWrapperImpl` class name is unfortunate. It makes sense to rename it into something more descriptive, for example `AvroFormatVisitorWrapper`.

Implementation in 2.13:

public class VisitorFormatWrapperImpl
implements JsonFormatVisitorWrapper

Citation from #292 comment:
> Maybe it would make sense to also file a follow-up issue about actual renaming (of `VisitorFormatWrapperImpl` ) like you suggested first (to `AvroFormatVisitorWrapper`).
> I'd have to think if it actually could go in 2.14, after all, or at least in 3.0.

Question is, in which release?