Add UserDiscoveryConfig model

Sources/ElixxirDAppsSDK/Models/UserDiscoveryConfig.swift

+import Foundation
+
+public struct UserDiscoveryConfig: Equatable {
+  public init(cert: Data, address: Data, contact: Data) {
+    self.cert = cert
+    self.address = address
+    self.contact = contact
+  }
+
+  public var cert: Data
+  public var address: Data
+  public var contact: Data
+}