feat(optimizer): coalesce peer first_value / last_value into a single struct aggregate

[tohuya6]

Which issue does this PR close?

Closes #23602.
Part of the epic #23600.

Depends on #23628 (Closes #23601). This rule produces a Struct-typed first_value
argument, which only takes the fast GroupsAccumulator path once #23628 merges. On current
main that fast path doesn't exist for Struct, so enabling the flag is a regression — the
config therefore defaults to false, and the speedup is not observable until #23628 lands.

Rationale for this change

SELECT ..., first_value(a ORDER BY o), first_value(b ORDER BY o), ... GROUP BY p plans one
independent first_value / last_value aggregate per selected column. When those peers all
share the same ORDER BY, they pick the same winning row for every column, so the work is
duplicated across N expressions even though a single pass already determines the winner for all.

Two costs on wide "one row per group" payloads:

• Runtime. Each first_value runs its own argmax pass over the input, comparing the
  ORDER BY key row by row. Selecting N winner columns is N independent scans of the same rows,
  so cost scales linearly with the number of peer expressions.
• State. Each aggregate holds its own per-group accumulator slot, so the plan keeps N slots
  per group — on wide payloads, N copies of the winner's columns retained for every group key.

What changes are included in this PR?

Adds a logical optimizer rule CoalesceFirstLast (datafusion/optimizer/src/coalesce_first_last.rs)
that runs bottom-up over Aggregate nodes. Within a single Aggregate, it groups peer
first_value / last_value expressions by (function, ORDER BY, null treatment) and, for any
bucket with two or more members, rewrites them into one struct-valued aggregate whose argument is
a named_struct of the original values. A Projection on top unpacks the struct back into the
original columns with get_field, so the rewrite is transparent to everything above the Aggregate:

Aggregate: groupBy=[[p]], aggr=[[first_value(a ORDER BY o), first_value(b ORDER BY o)]]

becomes

Projection: p, get_field(s, 'c0') AS first_value(a ...), get_field(s, 'c1') AS first_value(b ...)
  Aggregate: groupBy=[[p]], aggr=[[first_value(named_struct('c0', a, 'c1', b) ORDER BY o) AS s]]

• Coalesces only within a bucket keyed by (function, ORDER BY, null treatment); buckets with
  fewer than two members are left unchanged.
• Bails on DISTINCT / FILTER / IGNORE NULLS, grouping-set aggregates
  (GROUPING SETS / ROLLUP / CUBE), and expressions without an ORDER BY.
• No-ops when named_struct / get_field are not in the function registry.
• Gated behind a new config optimizer.enable_coalesce_first_last, default false.
• Registers the new config/rule in the generated artifacts: configs.md,
  information_schema.slt, optimizer_rule_reference.md, and the EXPLAIN VERBOSE list in
  explain.slt.

Are these changes tested?

Yes.

• Unit tests in coalesce_first_last.rs asserting the rewritten plan shape (peer first_value
  collapsed into one named_struct aggregate + get_field projection; same for last_value; a
  non-coalesced aggregate such as count preserved alongside), plus no-op cases: a single
  first_value, mixed first_value + last_value, DISTINCT, a grouping-set aggregate, flag
  disabled, missing registry.
• An end-to-end sqllogictest (coalesce_first_last.slt) that runs the query with the flag off and
  on over multi-group data containing NULLs and asserts identical results, plus an EXPLAIN
  pinning the rewritten logical and physical plans.
• cargo fmt, cargo clippy --all-targets --all-features -- -D warnings, the full sqllogictest
  suite, and the config/rule doc-generation checks all pass.

Are there any user-facing changes?

Adds one new configuration option, datafusion.optimizer.enable_coalesce_first_last, defaulting
to false. No SQL or API surface changes: with the flag off the plan is unchanged, and with it on
the rewrite is result-preserving, so existing queries and previously-passing tests behave identically.

Follow-ups

• Once feat(functions-aggregate): support nested types (List, Struct, Map) in first_value / last_value GroupsAccumulator #23628 (Closes Extend first_value / last_value GroupsAccumulator to nested types (List, Struct, Map) #23601) merges, consider flipping the default after benchmarking across
  payload widths and group cardinalities.
• Logical rewrite: Filter(row_number() = 1) over PARTITION BY → Aggregate(FIRST_VALUE(... ORDER BY)) #23603 (logical rewrite Filter(row_number() = 1) → Aggregate(FIRST_VALUE(... ORDER BY)))
  composes with this rule: the peer FIRST_VALUE aggregates it emits would be coalesced here.