add proc transport option

[feymartynov]

A use case for it is setting up a connection to AWS Elasticsearch.

According to elasticsearch-transport's readme one should pass a block to ::ElasticsearchClient#initialize which adds necessary authorization headers to each request. However this can't be done with chewy.

A possible hack as it was discussed in the issue #296 is to assign Thread.current[:chewy_client] somewhere (say, in Rails initializer) with a custom instance. But this would affect only the current thread.

With :proc option it becomes possible to pass a block to Elasticsearch::Constructor.

So an example initializer for AWS tuning looks like this:

require 'faraday_middleware/aws_signers_v4'

Chewy.configuration[:transport_options] = {
  headers: { content_type: 'application/json' },
  proc: -> (f) do
      f.request :aws_signers_v4,
                service_name: 'es',
                region: 'us-east-1',
                credentials: Aws::Credentials.new(
                  ENV['AWS_ACCESS_KEY'],
                  ENV['AWS_SECRET_ACCESS_KEY'])
  end
}

P.S. For the purpose of Content-Type header see issue #271.

[pyromaniac]

Thanks!

[rromanchuk]

o_O am i on an incorrect version? or is there something fundamental i'm not understanding here?