Add ESQL DAY_NAME function (#132535)

* Add DayName function

* Add reference list of date functions in docs

* Add ESQLCapabilities gate

* Add locale specific tests

* Improve docs

* Improve formatting

* Remove trailing blank line

* simplify code

* change capability name to keep consistent

* update tests

* add info about GA on 9.2.0

Author: romulets

docs/reference/query-languages/esql/_snippets/functions/description/day_name.md

@@ -3,4 +3,5 @@
 * [`DATE_FORMAT`](../../functions-operators/date-time-functions.md#esql-date_format)
 * [`DATE_PARSE`](../../functions-operators/date-time-functions.md#esql-date_parse)
 * [`DATE_TRUNC`](../../functions-operators/date-time-functions.md#esql-date_trunc)
+* [`DAY_NAME`](../../functions-operators/date-time-functions.md#esql-day_name)
 * [`NOW`](../../functions-operators/date-time-functions.md#esql-now)

docs/reference/query-languages/esql/_snippets/functions/examples/day_name.md

@@ -28,6 +28,9 @@ mapped_pages:
 :::{include} ../_snippets/functions/layout/date_trunc.md
 :::
 
+:::{include} ../_snippets/functions/layout/day_name.md
+:::
+
 :::{include} ../_snippets/functions/layout/now.md
 :::