Makes SelfDescribingJson class open to allow for inheritance (#906)

This allow inheritance on the client side. This as the behaviour available with Objc version of the SDK and one I could not find any drawback in the current state of the SDK. Being able to inherit allows of use of type's init while preserving SelfDescribingJson identity.

Author: TwoDollarsEsq

Sources/Snowplow/Payload/SelfDescribingJson.swift

@@ -18,7 +18,7 @@ import Foundation
 /// This class holds the information of a self-describing JSON.
 /// - seealso: SPPayload
 @objc(SPSelfDescribingJson)
-public class SelfDescribingJson: NSObject {
+open class SelfDescribingJson: NSObject {
     /// the schema URI for this self-describing JSON.
     @objc
     public var schema: String