Add PasswordStorage

Sources/ElixxirDAppsSDK/Errors.swift

@@ -5,3 +5,7 @@ public struct BindingsDownloadAndVerifySignedNdfWithUrlUnknownError: Error, Equa
 public struct BindingsGenerateSecretUnknownError: Error, Equatable {
   public init() {}
 }
+
+public struct PasswordStorageMissingPasswordError: Error, Equatable {
+  public init() {}
+}

Sources/ElixxirDAppsSDK/PasswordStorage.swift

+import Foundation
+
+public struct PasswordStorage {
+  public init(
+    save: @escaping (Data) throws -> Void,
+    load: @escaping () throws -> Data
+  ) {
+    self.save = save
+    self.load = load
+  }
+
+  public var save: (Data) throws -> Void
+  public var load: () throws -> Data
+}
+
+#if DEBUG
+extension PasswordStorage {
+  public static let failing = PasswordStorage(
+    save: { _ in
+      struct NotImplemented: Error {}
+      throw NotImplemented()
+    },
+    load: {
+      struct NotImplemented: Error {}
+      throw NotImplemented()
+    }
+  )
+}
+#endif