feat: add support for _partition metadata column

[parthchandra]

Which issue does this PR close?

• part of #Support for projecting metadata columns _pos, _spec_id, and _partition in table scan #2607.

What changes are included in this PR?

Implements the _partition metadata column for table scans. This is a struct column whose type is the union of all partition fields across all partition specs (handling partition evolution). Each row gets the
partition values for its data file.

• Adds compute_unified_partition_type() to compute the union of partition fields across all specs (equivalent to Java's Partitioning.partitionType())
• Adds PartitionColumnConstant and build_partition_column_constant() for pre-computing the struct values per file
• Adds ColumnSource::AddStructConstant variant to RecordBatchTransformer for materializing struct columns
• Threads the unified partition type through scan planning and populates the constant in FileScanTask
• Pipeline detects RESERVED_FIELD_ID_PARTITION in projected fields and injects the struct constant

Are these changes tested?

Because we do not have write support yet, I made the corresponding change to comet and then tested by adding tests in Comet which uses iceberg-java to write files and then iceberg-rust to read them back.
https://github.com/parthchandra/datafusion-comet/blob/iceberg-metadata-columns/spark/src/test/resources/sql-tests/iceberg/metadata_column_partition.sql

[parthchandra]

@advancedxy fyi