add geo_centroid metric aggregation

[nknize]

Brought up in discussion of PR #13433 this feature moves the weighted centroid result from geohash_grid_aggregator a its own geo_centroid metric aggregation. This will save space for users that do not care or need the weighted centroid, and decouple the calculation such that it can be used as a subaggregation. This allows the weighted centroid to be calculated based on attributes besides just geolocation (e.g., weights based on population density or median household income)