Add no-internet connection monitoring

[jjorn]

Description

This PR implements a monitoring concept for availalbe internet connection.

Related Issue

This PR should also solve #43

Changes:

• Add a link to built-in SystemConfiguration framework
• Implement NetworkReachabilityManager acting as a monitor of current internet connection
• Add Client.Task as a wrapper for the client operations - request, upload and download
• Add ClientTaskExecutor including network connection monitoring

Notes

The feature of reachability has been built on SystemConfiguration framework. As an alternative, we can also use NWPathMonitor in Network framework which is available from iOS 12.0.

[JensK611]

@jjorn What's the actual state of this PR?