(WIP) Implement geo_grid aggregation

[nyurik]

This PR has been superseded by #37842

Reworked #30320 PR

This PR adds a new geo_grid aggregation designed to eventually replace the geohash_grid aggregation. It has identical parameters as geohash_grid, but also requires a hash_type parameter to indicate which algorithm to use. There is no defaults.

The first PR adds the geohash algorithm, but a follow up will add the tile-based (quadkey - #30240 )

Minor interface change: getKey() and getKeyAsString() return the same string for the new type of bucket.

TODO

• Fix ParsedGeoGrid

Changes from #30320

• Do not refactor existing geohash_grid, but instead add a new geo_grid (copies most of the existing code), and deprecate geohash_grid.
• geo_grid will require hash_type parameter. Setting it to "geohash" makes it behave the same as the geohash_grid.
• hash_type is implemented as a string instead of an Enum. This allows for a more flexible addition of the new algorithms.
• Each bucket no longer contains a type, making bucket transmission cost the same as for the geohash_grid

[elasticmachine]

Pinging @elastic/es-analytics-geo

[nyurik]

Done in #37842