ESQL: Speed up VALUES for many buckets (elastic#123073)

Speeds up the VALUES agg when collecting from many buckets.
Specifically, this speeds up the algorithm used to `finish` the
aggregation. Most specifically, this makes the algorithm more tollerant
to large numbers of groups being collected. The old algorithm was
`O(n^2)` with the number of groups. The new one is `O(n)`

```
(groups)
      1     219.683 ±    1.069  ->   223.477 ±    1.990 ms/op
   1000     426.323 ±   75.963  ->   463.670 ±    7.275 ms/op
 100000   36690.871 ± 4656.350  ->  7800.332 ± 2775.869 ms/op
 200000   89422.113 ± 2972.606  -> 21920.288 ± 3427.962 ms/op
 400000 timed out at 10 minutes -> 40051.524 ± 2011.706 ms/op
```

The `1` group version was not changed at all. That's just noise in the
measurement. The small bump in the `1000` case is almost certainly worth
it and real. The huge drop in the `100000` case is quite real.

benchmarks/src/main/java/org/elasticsearch/benchmark/compute/operator/AggregatorBenchmark.java

@@ -60,6 +60,9 @@
 import java.util.stream.LongStream;
 import java.util.stream.Stream;
 
+/**
+ * Benchmark for many different kinds of aggregator and groupings.
+ */
 @Warmup(iterations = 5)
 @Measurement(iterations = 7)
 @BenchmarkMode(Mode.AverageTime)

docs/changelog/123073.yaml

@@ -0,0 +1,5 @@
+pr: 123073
+summary: Speed up VALUES for many buckets
+area: ES|QL
+type: bug
+issues: []