Add LoadOrNewUserDiscovery functor

Sources/ElixxirDAppsSDK/Functors/LoadOrNewUserDiscovery.swift

+import Bindings
+import XCTestDynamicOverlay
+
+public struct LoadOrNewUserDiscovery {
+  public var run: (Int, UdNetworkStatus, String, Data) throws -> UserDiscovery
+
+  public func callAsFunction(
+    e2eId: Int,
+    follower: UdNetworkStatus,
+    username: String,
+    registrationValidationSignature: Data
+  ) throws -> UserDiscovery {
+    try run(e2eId, follower, username, registrationValidationSignature)
+  }
+}
+
+extension LoadOrNewUserDiscovery {
+  public static let live = LoadOrNewUserDiscovery {
+    e2eId, follower, username, registrationValidationSignature in
+
+    var error: NSError?
+    let bindingsUD = BindingsLoadOrNewUserDiscovery(
+      e2eId,
+      follower.makeBindingsUdNetworkStatus(),
+      username,
+      registrationValidationSignature,
+      &error
+    )
+    if let error = error {
+      throw error
+    }
+    guard let bindingsUD = bindingsUD else {
+      fatalError("BindingsLoadOrNewUserDiscovery returned `nil` without providing error")
+    }
+    return .live(bindingsUD)
+  }
+}
+
+extension LoadOrNewUserDiscovery {
+  public static let unimplemented = LoadOrNewUserDiscovery(
+    run: XCTUnimplemented("\(Self.self)")
+  )
+}