Support for Geohash as bounding box in geo_bounding_box

[imotov]

Adds support for specifying the bounding box in the geo_bounding_box
query using a single geohash. It is already possible to use geohashes
as corners, this change adds support to using a single geohash as a
complete bounding box.

Closes #25154

[elasticmachine]

Pinging @elastic/es-search-aggs

[jpountz]

My initial expectation was that we would accept a geo hash as the top left and bottom right locations, but fix the parsing to take the top-left corner of the top-left geohash cell and the bottom right corner of the bottom-right geohash cell. I hadn't considered adding an option to pass a single geohash directly, I need to think more about the pros/cons. If we go that path, we should probably still fix the parsing of geohashes as top-left / bottom-right coordinates?

[imotov]

If we go that path, we should probably still fix the parsing of geohashes as top-left / bottom-right coordinates?

I am not sure what we need to fix there. It looked to me that we support geohashes as top-left / bottom-right, but we treat them as points, no as bounding boxes in this case.

[jpountz]

It looked to me that we support geohashes as top-left / bottom-right, but we treat them as points, no as bounding boxes in this case.

I was considering the case that someone uses a geohash that has a short length as a corner. In that case we should try to only consider the point from the geohash that makes most sense? Eg. the top-left corner of the geohash if the geohash is used as a top-left corner, and the bottom right corner of the geohash if the geohash is used as a bottom right corner?

[imotov]

Superseded by #30698