[ES|QL] Add a standard deviation function (elastic#116531)

Uses Welford's online algorithm, as well as the parallel version, to
calculate standard deviation.

Author: limotova

docs/changelog/116531.yaml

@@ -0,0 +1,5 @@
+pr: 116531
+summary: "Add a standard deviation aggregating function: STD_DEV"
+area: ES|QL
+type: enhancement
+issues: []

docs/reference/esql/functions/aggregation-functions.asciidoc

@@ -17,6 +17,7 @@ The <<esql-stats-by>> command supports these aggregate functions:
 * <<esql-min>>
 * <<esql-percentile>>
 * experimental:[] <<esql-st_centroid_agg>>
+* <<esql-std_dev>>
 * <<esql-sum>>
 * <<esql-top>>
 * <<esql-values>>
@@ -32,6 +33,7 @@ include::layout/median_absolute_deviation.asciidoc[]
 include::layout/min.asciidoc[]
 include::layout/percentile.asciidoc[]
 include::layout/st_centroid_agg.asciidoc[]
+include::layout/std_dev.asciidoc[]
 include::layout/sum.asciidoc[]
 include::layout/top.asciidoc[]
 include::layout/values.asciidoc[]