Added an option to add arbitrary headers to the client requests

The headers are key/value pairs defined in the settings under the `request.headers` namespace.

Also changed test infrastructure's so it'd be possible to define the additional settings for transport clients.
Regarding the testing infra extension, the one thing I don't get is where we would need to use it. If it's needed in some tests (other than this) I'm +1 on adding it, otherwise I don't quite see the point. Am I missing something?