feat: add support for _spec_id metadata column

[hsiang-c]

Which issue does this PR close?

• Partially closes Support for projecting metadata columns _pos, _spec_id, and _partition in table scan #2607

What changes are included in this PR?

• If a projection includes _spec_id, which is a constant like the _file metadata column for all rows, add it to RecordBatchTransformerBuilder
• Fix the partition value of manifest entry. The reason is we have the following partitions spec in crates/iceberg/testdata/example_table_metadata_v2.json and the x column is always 1 for all data files, so the identity partition transform value should be 1.

  "default-spec-id": 0,
  "partition-specs": [
    {
      "spec-id": 0,
      "fields": [
        {"name": "x", "transform": "identity", "source-id": 1, "field-id": 1000}
      ]
    }
  ],

Are these changes tested?

• Yes, unit tests.

[hsiang-c]

FYI @advancedxy @parthchandra

[advancedxy]

I think we should always add spec_id into the constant map if it's selected/projected.

The task.partition_spec should be present even for unpartitioned table(spec_id = 0).

[advancedxy]

the partition spec is already in mainifest_file, like the partition_spec_id. maybe we could just use that and avoid embedding partition_spec into ManifestFileContext

[advancedxy]

@hsiang-c thanks, I think it's in good shape, just two minor comments.