feat(core): add MaskExpression POJO and ReadRel projection support

[flex-seongmin]

Summary

Implement the MaskExpression POJO layer for ReadRel projection support in substrait-java.

The Substrait protobuf spec defines Expression.MaskExpression as a field on ReadRel (field 4: projection), but the Java POJO layer had only a TODO comment and no actual implementation. This means any ReadRel projection information was silently dropped during proto-to-POJO conversion, and POJO-constructed plans could not express column projection at the read level.

This change closes that gap by introducing a full POJO model for MaskExpression and wiring it through the existing conversion infrastructure.

Changes

• MaskExpression.java (new) -- @Value.Enclosing interface with 11 @Value.Immutable inner types (MaskExpr, Select, StructSelect, StructItem, ListSelect, ListSelectItem, ListElement, ListSlice, MapSelect, MapKey, MapKeyExpression) matching the proto definition 1:1
• MaskExpressionProtoConverter.java (new) -- Dedicated bidirectional converter between io.substrait.proto.Expression.MaskExpression and the POJO types, keeping proto FQN references isolated from the relation converters
• AbstractReadRel.java -- Replaced TODO comment with Optional<MaskExpression.MaskExpr> getProjection()
• ProtoRelConverter.java -- Added optionalMaskExpression() helper; all four ReadRel subtypes (NamedScan, LocalFiles, VirtualTableScan, ExtensionTable) now populate projection during proto-to-POJO conversion
• RelProtoConverter.java -- All four visit(ReadRel) methods now serialize projection back to proto via MaskExpressionProtoConverter.toProto()

Testing

Added 11 roundtrip tests in ReadRelRoundtripTest covering:

• Simple flat column projection (StructSelect only)
• Nested struct projection (StructItem with child Select.ofStruct)
• List element and slice selection (ListSelect)
• List selection with nested child (ListSelect + child StructSelect)
• Map key selection (MapSelect with MapKey)
• Map key expression / wildcard selection (MapKeyExpression)
• Map selection with nested child (MapSelect + child StructSelect)
• maintainSingularStruct flag
• Projection combined with filter
• Projection on VirtualTableScan and LocalFiles

[CLAassistant]

All committers have signed the CLA.