feat: SortRel mlir implementation

[xwkuang5]

This commit adds support for the Substrait SortRel relation in the MLIR dialect.

Key Changes:

• Operations: Added substrait.sort and substrait.sort_field_compare operations to SubstraitOps.td.
• Enums: Added SortFieldComparisonType to SubstraitEnums.td to represent standard sort directions (e.g., ASC_NULLS_FIRST, DESC_NULLS_LAST).
• Import Logic: Implemented importSortRel in Import.cpp. This uses a temporary dummy region to instantiate the sort expression once, which is then cloned using IRMapping to populate the comparison block for both the LHS and RHS rows.
• Export Logic: Implemented exportOperation for SortOp in Export.cpp to generate the corresponding SortRel protobuf message.
• Verification: Added SortOp::verifyRegions to ensure the sort region is well-formed (blocks take two arguments and yield si8).

Supported Features:

• All standard sort directions defined in the Substrait specification (ASC_NULLS_FIRST, ASC_NULLS_LAST, DESC_NULLS_FIRST, DESC_NULLS_LAST, CLUSTERED).
• Sorting by arbitrary expressions supported by the dialect (field references, literals, etc.).

Limitations:

• Custom Comparison Functions: comparison_function_reference in SortField is not yet supported. The import process will emit an error if this field is set.

Tests:

• Added test/Target/SubstraitPB/Import/sort.textpb
• Added test/Target/SubstraitPB/Export/sort.mlir

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