User Discovery Environment

• Add UDEnvironment struct that wraps ud address, cert, and contact into a single model.
• Use UDEnvironment in XXMessengerClient

Sources/XXClient/Models/UDEnvironment.swift

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