sql: implement the any_value() aggregate function

[virajchogle]

any_value() was added in PostgreSQL 16 and returns an arbitrary non-NULL value from the input group, or NULL if there are no non-NULL values. See #172590

Implementation

CockroachDB already has an internal aggregate with exactly these semantics: any_not_null, used by the optimizer to pass through columns that are constant within a group. This PR exposes it under the PostgreSQL name rather than duplicating it: the builtin reuses the any_not_null constructor, and the optimizer lowers any_value to the existing AnyNotNullAgg operator, following the precedent of aliases like every/bool_and. Vectorized and distributed execution work without introducing a new AggregatorSpec function.

One consequence is that EXPLAIN shows any-not-null-agg for the aggregation (the output column keeps the any_value name). Happy to switch to a dedicated operator if that is preferred.

Latent window bug

any_value(x) OVER (...) initially crashed with an internal error: the vectorized planner assumes every optimized aggregate can run as a window aggregate, but any_not_null is the only one without a generated window variant. This was unreachable before since users had no way to name any_not_null. The Windower support check now rejects it so these queries fall back to the row-based windower.

Tests

Logic tests cover NULL skipping, all-NULL group, empty input, GROUP BY, FILTER, DISTINCT, and window usage; all logictest configs pass, including mixed-version. Optbuilder testdata locks in the lowering.

Resolves: #172590
Epic: none

Release note (sql change): Added the any_value() aggregate function, which returns an arbitrary non-NULL value from the input group, matching PostgreSQL 16.

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