iOS: Refactor download mechanics to allow future auth headers to be added

[davidovich]

It would be nice to allow auth headers to be added to the network operations that the plugin initiates. This would allow protected uris to serve the chcp content.

To allow authentication headers to be added, we need to refactor downloading mechanics to use a single method of downloading files, which can be configured with credentials, thus allowing authentication. This issue is opened to track development of such a feature.

Having a way to customize all headers is an open question. But initially, to not put too much burden on the users side, we will concentrate on specifying credentials. Other custom headers could be added in the future.

Another issue will be opened to track and discuss the user facing (js) api.