[ES|QL] add reverse function (elastic#113297) (elastic#114163)

Adds a REVERSE string function

Author: drewdaemon

docs/changelog/113297.yaml

@@ -0,0 +1,5 @@
+pr: 113297
+summary: "[ES|QL] add reverse function"
+area: ES|QL
+type: enhancement
+issues: []

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

@@ -17,6 +17,7 @@
 * <<esql-ltrim>>
 * <<esql-repeat>>
 * <<esql-replace>>
+* <<esql-reverse>>
 * <<esql-right>>
 * <<esql-rtrim>>
 * <<esql-space>>
@@ -38,6 +39,7 @@ include::layout/locate.asciidoc[]
 include::layout/ltrim.asciidoc[]
 include::layout/repeat.asciidoc[]
 include::layout/replace.asciidoc[]
+include::layout/reverse.asciidoc[]
 include::layout/right.asciidoc[]
 include::layout/rtrim.asciidoc[]
 include::layout/space.asciidoc[]