Avro: Support timestamp(9) and unknown types

[rdblue]

This updates Avro to support nanosecond timestamps and unknown.

This also cleans up some of the tests:

• Moves DataTest to core so it can be used for Avro, rather than duplicating in AvroDataTest
• Enables default value tests from DataTest for the internal object model (TestInternalAvro)
• To use DataTest for TestInternalAvro, uses Record instead of StructLike and updates validations in InternalTestHelpers

The new tests in DataTest are currently disabled for Parquet, ORC, and Flink that reuse the DataTest base using supportsUnknown, supportsTimestampNanos, and supportsVariant. This allows breaking up the changes to support these types across multiple PRs.

[danielcweeks]

+1 This all looks good to me. The only thing I note is that we're changing the terminology a little (opiton -> optional). While I agree with the using optional as it is less confusing, we're slightly inconsistent.

[rdblue]

The only thing I note is that we're changing the terminology a little (option -> optional). While I agree with the using optional as it is less confusing, we're slightly inconsistent.

The distinction I was going for was between an optional field (may be null) and an option schema (a union of null and another type). I know it's a bit weird, but I didn't want to rename all of the uses of the latter meaning because we have isOptionSchema, toOption, and fromOption methods. I think this is the cleanest option.

[rdblue]

Thanks for reviewing, @danielcweeks!