Avro adapter: AvroNullableConsumer should use readIndex

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

When using Avro's `ValidatingDecoder`, any schema with a nullable field fails with the following exception:

```
Exception in thread "main" org.apache.avro.AvroTypeException: Attempt to process a int when a union was expected.
at org.apache.avro.io.parsing.Parser.advance(Parser.java:91)
at org.apache.avro.io.ValidatingDecoder.readInt(ValidatingDecoder.java:81)
at org.apache.arrow.adapter.avro.consumers.AvroNullableConsumer.consume(AvroNullableConsumer.java:44)
```

Reason: `AvroNullableConsumer` uses `readInt` to read the index, but it should use `readIndex` instead. It's irrelevant in case of a `BinaryDecoder`, for example (`readIndex` is the same as `readInt`), but `ValidatingDecoder`s and any Decoders based on that have different logic.

Start in AvroNullableConsumer.consume, at the readInt call identified in the stack trace, and compare it with Avro's decoder API and ValidatingDecoder behavior. Done means nullable fields can be consumed with a ValidatingDecoder without the union-versus-int exception.

Written by the indexing model from the issue text.