Missing retriever types

[flobernd]

We are currently missing the rescorer and linear retriever types.

See:
https://www.elastic.co/docs/reference/elasticsearch/rest-apis/retrievers

Related to:
elastic/elasticsearch-net#8528

[l-trotta]

we're also missing pinned and random rank

[sergey-tihon]

we're also missing pinned and random rank

why they are not mentioned in docs? https://www.elastic.co/docs/reference/elasticsearch/rest-apis/retrievers

[l-trotta]

I think they're only available through plugins

[leemthompo]

@Samiul-TheSoccerFan @Mikep86 do you folks have a corresponding issue for this? I'm trying to track down various open threads about improving the retrievers docs