Bindings models wrapper

• Add Identity model.
• Add MessageSendReport model.
• Add Fact model.
• Add Message model.
• Add RestlikeMessage model.
• Add tests for models decoding and encoding (use example JSONs from Bindings as expected JSON schemas).

Sources/ElixxirDAppsSDK/Fact.swift

+public struct Fact: Equatable {
+  public init(
+    fact: String,
+    type: Int
+  ) {
+    self.fact = fact
+    self.type = type
+  }
+
+  public var fact: String
+  public var type: Int
+}
+
+extension Fact: Codable {
+  enum CodingKeys: String, CodingKey {
+    case fact = "Fact"
+    case type = "Type"
+  }
+}