Add support for geotile_grid aggregations

[owrcasstevens]

Currently using version 7.1 of elasticsearch-dsl, but geotile_grid aggregations do not seem to be supported. When I do:

grid_agg = search.aggs.bucket("grid", "geotile_grid", field="centroid", precision=2)

I am getting:

raise UnknownDslObject('DSL class `{}` does not exist in {}.'.format(name, cls._type_name))

Digging into the code a bit, it seems that the following aggs are supported:

filter
filters
children
parent
date_histogram
auto_date_histogram
date_range
geo_distance
geohash_grid
geo_centroid
global
histogram
ip_range
missing
nested
range
reverse_nested
significant_terms
significant_text
terms
sampler
diversified_sampler
composite
top_hits
avg
weighted_avg
cardinality
extended_stats
geo_bounds
max
min
percentiles
percentile_ranks
scripted_metric
stats
sum
value_count
avg_bucket
bucket_script
bucket_selector
cumulative_sum
derivative
extended_stats_bucket
max_bucket
min_bucket
moving_fn
moving_avg
percentiles_bucket
serial_diff
stats_bucket
sum_bucket
bucket_sort

But geotile_grid is not in there, which is surprising since Kibana switched to using geotile_grid aggs for maps:
elastic/kibana#30417

[sethmlarson]

Agreed this would be great to add. My dev time is being spent elsewhere currently but eventually I'll get to this.

In the meantime if you're up to it I can review a submitted PR :)

[mbarretta]

@owrcasstevens is the lower-level elasticsearch-py and option? I'm not a python guy, but it seems you could use the search method to form up the geotile_grid agg in json.

...and just noticed you already submitted a PR. Nevermind!