ES|QL: Add TBUCKET function (#131449)

Introduce the function TBUCKET(time_interval) which applies grouping
on the @timestamp field, truncating its value to the specified
granularity:

TBUCKET(1h) is equivalent to BUCKET(1 hour, @timestamp)
TBUCKET(7d) is equivalent to BUCKET(7 days, @timestamp)

Closes #131068

Author: leontyevdv

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

@@ -1,2 +1,3 @@
 * [`BUCKET`](../../functions-operators/grouping-functions.md#esql-bucket)
+* [`TBUCKET`](../../functions-operators/grouping-functions.md#esql-tbucket)
 * [`CATEGORIZE`](../../functions-operators/grouping-functions.md#esql-categorize)

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

@@ -16,7 +16,10 @@ The [`STATS`](/reference/query-languages/esql/commands/stats-by.md) command supp
 :::{include} ../_snippets/functions/layout/bucket.md
 :::
 
-:::{note} 
+:::{include} ../_snippets/functions/layout/tbucket.md
+:::
+
+:::{note}
 The `CATEGORIZE` function requires a [platinum license](https://www.elastic.co/subscriptions).
 :::