Support authentication on the level of ElasticClient

[igor-vovk]

Addresses https://github.com/Philippus/elastic4s/issues/3122. I copied authentication settings that were introduced in http4s PR https://github.com/Philippus/elastic4s/pull/3118, will be happy to remove them from http4s PR after this will be merged.

Also, they are now can be removed from AkkaHttpClient, that also implements adding credentials by itself.

By defining credentials on the ElasticClient level there is now a single place where credentials are used, so there is no need to rely on client-specific implementations of credential passing.