Add a standard deviation function

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

Author: limotova

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_deviation>>
 * <<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_deviation.asciidoc[]
 include::layout/sum.asciidoc[]
 include::layout/top.asciidoc[]
 include::layout/values.asciidoc[]

docs/reference/esql/functions/description/std_deviation.asciidoc

@@ -608,6 +608,27 @@ tasks.named('stringTemplates').configure {
     it.outputFile = "org/elasticsearch/compute/aggregation/RateDoubleAggregator.java"
   }
 
+  File stdDeviationAggregatorInputFile = file("src/main/java/org/elasticsearch/compute/aggregation/X-StdDeviationAggregator.java.st")
+  template {
+    it.properties = intProperties
+    it.inputFile = stdDeviationAggregatorInputFile
+    it.outputFile = "org/elasticsearch/compute/aggregation/StdDeviationIntAggregator.java"
+  }
+  template {
+    it.properties = longProperties
+    it.inputFile = stdDeviationAggregatorInputFile
+    it.outputFile = "org/elasticsearch/compute/aggregation/StdDeviationLongAggregator.java"
+  }
+  template {
+    it.properties = floatProperties
+    it.inputFile = stdDeviationAggregatorInputFile
+    it.outputFile = "org/elasticsearch/compute/aggregation/StdDeviationFloatAggregator.java"
+  }
+  template {
+    it.properties = doubleProperties
+    it.inputFile = stdDeviationAggregatorInputFile
+    it.outputFile = "org/elasticsearch/compute/aggregation/StdDeviationDoubleAggregator.java"
+  }
 
   File topAggregatorInputFile = new File("${projectDir}/src/main/java/org/elasticsearch/compute/aggregation/X-TopAggregator.java.st")
   template {