Add Point in Time API functionality

[Ryan3435]

Please use the following questions as a guideline to help me answer
your issue/question without further inquiry. Thank you.

Which version of Elastic are you using?

[X] elastic.v7 (for Elasticsearch 7.x)
[ ] elastic.v6 (for Elasticsearch 6.x)
[ ] elastic.v5 (for Elasticsearch 5.x)
[ ] elastic.v3 (for Elasticsearch 2.x)
[ ] elastic.v2 (for Elasticsearch 1.x)

Please describe the expected behavior

Elasticsearch seems to recommend moving away from the Scroll API for deep pagination and recommends using the search_after parameter combined with a point in time (PIT) for preserving index state as seen here https://www.elastic.co/guide/en/elasticsearch/reference/current/scroll-api.html

The point in time API is a newer feature that doesn't seem to be implemented in this package yet but has been implemented in the Java API elastic/elasticsearch#61062

The rest APIs for opening and closing PITs can be found here:
https://github.com/elastic/elasticsearch-net/blob/7.10/src/ApiGenerator/RestSpecification/XPack/close_point_in_time.json
https://github.com/elastic/elasticsearch-net/blob/7.10/src/ApiGenerator/RestSpecification/XPack/open_point_in_time.json

and the reference for the Point in Time API is here https://www.elastic.co/guide/en/elasticsearch/reference/current/point-in-time-api.html

I would like to get a jump on moving my code over to using this method rather than scrolling if this is the new recommendation. If you are accepting of pull requests I can work on adding this functionality.

Please describe the actual behavior

Doesn't seem to be implemented yet.

Any steps to reproduce the behavior?

N/A

[olivere]

Initial support will be added to 7.0.24.