Improve syncthing REST client with Swift Codable

[xor-gate]

Currently syncthing daemon HTTP REST calls are handbrewed, we should map them to native objects which is more convienent using https://github.com/RestKit/RestKit

[leszarna]

I am pretty sure handbrewed is better than this library because RestKit:

• it is too big and does too much,
• it depends on old AFNetworking
• it has maintenance problems, bugs and questionable design.
  My advise is to use newest Apple protocol named Codable (https://developer.apple.com/documentation/swift/codable). There is not much work with this and it's robust solution ready for
  future updates and improvements.

[xor-gate]

Thanks @leszarna for your comment.