Support Ref types in Avro Reader.

### Is your feature request related to a problem or challenge?

Currently Ref types are not supported at all, if the reader is set to infer the schema from the file, we'll enter an unreachable block, if we manually provide an Arrow schema, and the writer_schema has a Ref somewhere - it's worse, any Ref type will just appear as Null in the data without any explanation.

### Describe the solution you'd like

Refs should be supported, as far as arrow can support them.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Start with the Avro Reader and the writer_schema path described in the issue, then trace how Ref types are handled during schema inference and data reading. Done means supported Ref types no longer enter an unreachable block or silently appear as Null when an Arrow schema is provided.

Written by the indexing model from the issue text.