[TIMOB-18832] iOS: Implement NSURLSessionConfiguration to support HTTPAdditionalHeaders

Feature Request

Provide support to add a dictionary of [additional headers](https://developer.apple.com/library/mac/documentation/Foundation/Reference/NSURLSessionConfiguration_class/index.html#//apple_ref/occ/instp/NSURLSessionConfiguration/HTTPAdditionalHeaders) to send with requests. In native iOS, NSURLSessionConfiguration is a class that allows you to configure your NSURLSession with connection/HTTP policies, caching behavior, maximum number of connections, custom headers, etc. For example, if you have a scenario where an authentication token is required to be passed in header on every call to a RESTful API, you can set the token in the session configuration and it will apply to all requests made using that session.