Elasticsearch-net: Per field custom similarity in fluent PUT mapping

[lncramer]

Nest version: 2.1.1

The current fluent syntax for creating a PUT mapping forces you to use a built in SimilarityOption (either Default or BM25) when specifying a per field similarity (https://github.com/elastic/elasticsearch-net/blob/master/src/Nest/Mapping/Types/CorePropertyDescriptorBase.cs), but I would like to use a custom similarity specified in the create index request instead. As far as I can tell this is currently only possible in Nest using AllFieldDescriptor.Similarity, but I don't want to apply the custom similarity to the _all field. According to the Elastic docs a per field custom similarity should be valid.

[gmarz]

Thanks for reporting @lncramer. Sadly, fixing this in our 2.x branch would be a breaking change. Targeting 5.x for now unless we can think of a clever work around.

[Mpdreamz]

@lncramer I opened some PR's for this so provided @gmarz and/or @russcam agrees it will be fixed in the next 2.x release as well.