Add similarity module config in index resource

[jiahuijiang]

Add configuration to allow setting similarity config https://www.elastic.co/guide/en/elasticsearch/reference/current/index-modules-similarity.html

[jiahuijiang]

😅 @phillbaker fixed assigning to nested setting field. ran the test locally and confirmed the settings of the created index looks correct!

[jiahuijiang]

@phillbaker I think there was a flake running CI :(

[jiahuijiang]

@phillbaker are there plans to cut a release soon? :D