[comet-parquet-exec] Simplify schema logic for CometNativeScan

[mbutrovich]

The current logic takes the data schema and the required schema from the Java side (in the scan node) and:

1. Converts back to a Parquet schema (Thrift encoded)
2. Serializes it to the native side
3. Parses it to a schema descriptor
4. Converts that to an Arrow schema

This process is introducing conversion errors that are difficult to recover from (e.g. Timestamp(milli) -> INT96 -> Timestamp(nano)). This PR simplifies the schema serialization and conversion to native side, building on what @viirya did with the partition schema (thank you for the inspiration!).

In this PR, data schema and required schema are now serialized as Spark types. On the native side they are converted to Arrow types. We also now serialize more schema info (column names, nullability) than we did for just partition schema.

[mbutrovich]

Tests: succeeded 657, failed 99, canceled 1, ignored 47, pending 0