Support for ignore_unmapped in GeoDistance Sort query

[developersarm]

Can we please add support for ignore_unmapped in the _geo_distance sort query?
https://www.elastic.co/guide/en/elasticsearch/reference/6.8/search-request-sort.html#geo-sorting

I can create a PR if you agree.

[sudo-suhas]

A PR would be very much welcome 🙂. Please do call out the Elasticsearch version that introduced this feature in the doc comments.