* [Nereids](Variant) Implement variant type in Variant and support new sub column access method
The query SELECT v["a"]["b"] from simple_var WHERE cast(v["a"]["b"] as int) = 1
1. During the binding stage, the expression element_at(var, "xxx") is transformed into a SlotReference with a specified path. This conversion is tracked in the StatementContext, where the parent slot is the primary key and the paths are secondary keys. This structure, known as subColumnSlotRefMap in the StatementContext, helps to eliminate duplicates of the same slot derived from identical paths.
2. A new rule, BindSlotWithPaths, is introduced in the analysis stage. This rule is responsible for converting slots with paths into their respective slot suppliers. To ensure that slots with paths are correctly associated with the appropriate LogicalOlapScan, an additional mapping, slotToRelation, is added to the StatementContext. This mapping links the top-level slot to its corresponding relation (i.e., LogicalOlapScan). Consequently, subsequent slots with paths can determine the correct LogicalOlapScan to merge with and modify accordingly.
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