Add support for is_write_index

[octocat-mona]

Describe the feature:

Allow setting of is_write_index for Create Index and Index Aliases API's which can be used by the Rollover API.

This seems currently not supported:

await client.CreateIndexAsync("my-index", s => s
    .Aliases(a => a
        .Alias("my-alias", d => d
            .IsWriteIndex() // <-- Method doesn't exist, only Filter() etc
        )
    )
);

Call via ES API:

PUT my-index
{
  "aliases": {
    "my-alias": {
      "is_write_index": true
    }
  }
}

[russcam]

Thanks for opening @KPStolk. Alias operations already had an ability to set is_write_index, but this was not exposed on aliases for the create index API. I have opened #3704 to address

[russcam]

Closing this as #3704 has been merged and will be in the next release