Avro adapter: AvroStringConsumer should use readString instead of readBytes

### Describe the bug, including details regarding any error messages, version, and platform.

Currently, `AvroStringConsumer` calls `decoder.readBytes` instead of `decoder.readString`. This can lead to exceptions when using `ResolvingDecoder`:

```
org.apache.avro.AvroTypeException: Attempt to process a bytes when a string was expected.

at org.apache.avro.io.parsing.Parser.advance(Parser.java:91)
at org.apache.avro.io.ResolvingDecoder.readBytes(ResolvingDecoder.java:236)
at org.apache.arrow.adapter.avro.consumers.AvroStringConsumer.consume(AvroStringConsumer.java:41)
```

Start in AvroStringConsumer.java at the consume method and inspect how the decoder is used for Avro strings. Reproduce the ResolvingDecoder case described in the issue, then verify that the consumer handles string data without the reported AvroTypeException.

Written by the indexing model from the issue text.