[8.19] ES|QL Support for ST_GEOHASH, ST_GEOTILE and ST_GEOHEX (#125143) (#128938)

* ES|QL Support for ST_GEOHASH, ST_GEOTILE and ST_GEOHEX (#125143)

Added support for the three primary scalar grid functions:
* `ST_GEOHASH(geom, precision)`
* `ST_GEOTILE(geom, precision)`
* `ST_GEOHEX(geom, precision)`

As well as versions of these three that take an optional `geo_shape` boundary (must be a `BBOX` ie. `Rectangle`).

And also supporting conversion functions that convert the grid-id from long to string and back to long.

This work represents the core of the feature to support geo-grid aggregations in ES|QL.

* Remove license checking mechanism from tests (unsupported in 8.x)

* Add 8.x level generated docs

* Link new spatial functions into the docs

* Remove unused imports after removal of license level tests

* Fix list syntax

* Fix asciidoc links

Author: craigtaverner

docs/changelog/125143.yaml

@@ -0,0 +1,6 @@
+pr: 125143
+summary: "ES|QL Support for ST_GEOHASH, ST_GEOTILE and ST_GEOHEX"
+area: "ES|QL"
+type: enhancement
+issues:
+ - 123903