Support native `variant_explode` and `variant_explode_outer`

[peterxcli]

What is the problem the feature request solves?

Spark 4.x provides variant_explode and variant_explode_outer to turn a Variant object or array into rows. Comet does not support these generators, so queries must leave native execution:

SELECT id, e.* FROM t LATERAL VIEW variant_explode(v) e AS pos, key, value;
SELECT id, e.* FROM t LATERAL VIEW variant_explode_outer(v) e AS pos, key, value;

Spark defines the generated columns as pos INT NOT NULL, key STRING, and value VARIANT NOT NULL, and implements object/array iteration in VariantExplode. The exact SQL names are registered as generators in Spark's 4.1.3 function registry.

Describe the potential solution

Add a Spark-4-specific generator path that:

• iterates arrays by index and objects by their stored field order;
• emits Spark-compatible pos, nullable key, and Variant value columns;
• preserves the Variant result Field marker and canonical [value, metadata] storage for each emitted value;
• matches Spark behavior for scalar values, Variant null, SQL NULL, empty containers, and the outer form; and
• keeps unrelated Variant-bearing generators/operators on explicit fallback.

Add focused parity and native-plan tests for objects, arrays, nested values, empty containers, scalars, Variant/SQL nulls, Unicode keys, aliases, lateral-view syntax, and outer versus non-outer cardinality.

Additional context

Related work: #4295, #5407, #5425, and the general generator execution path.

Non-goals: Variant mutation, subfield pruning or pushdown, shuffle/spill, C2R, writing, Python transport, and Iceberg-specific work.

[peterxcli]

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