Avoid O(N^2) in VALUES with ordinals grouping (#130576) (#130779)

Using the VALUES aggregator with ordinals grouping led to accidental
quadratic complexity. Queries like FROM .. | STATS ... VALUES(field) ...
BY keyword-field are affected by this performance issue. This change
caches a sorted structure - previously used to fix a similar O(N^2)
problem when emitting the output block - during the merging phase of the
OrdinalGroupingOperator.

Author: dnhatn

docs/changelog/130576.yaml

@@ -0,0 +1,5 @@
+pr: 130576
+summary: Avoid O(N^2) in VALUES with ordinals grouping
+area: ES|QL
+type: bug
+issues: []