Implement the GIS function ST_PointFromGeoHash

The GIS function ST_PointFromGeoHash takes in input a geohash and returns a point where the x is the longitude and the y is the latitude.
The latitude is returned as a numeric value in the interval [180, -180]. The longitude is returned as a numeric value in the interval [90, -90].
If the argument is NULL, the return value is NULL. If the argument is invalid, an ER_GIS_INVALID_DATA is thrown.