Allow file reader to read without setting ReaderOptions::projection

Currently `ReaderOptions::projection` (https://github.com/apache/iceberg-cpp/blob/main/src/iceberg/file_reader.h#L111) is required to be set before opening a parquet/avro reader, otherwise an error will be returned complaining about this. It would be good to allow reading all columns from the file if this field is unset.

Start at src/iceberg/file_reader.h around ReaderOptions::projection and trace how parquet and avro readers are opened when it is unset. Verify that an unset projection reads all columns without the current error, using the relevant reader checks or tests in the repository.

Written by the indexing model from the issue text.