Add FilePartStatus model

Sources/ElixxirDAppsSDK/Models/FilePartStatus.swift

+public enum FilePartStatus: Equatable {
+  case partDoesNotExist
+  case unsent
+  case arrived
+  case received
+  case unknown(code: Int)
+}
+
+extension FilePartStatus {
+  public init(rawValue: Int) {
+    switch rawValue {
+    case let value where value < 0: self = .partDoesNotExist
+    case 0: self = .unsent
+    case 1: self = .arrived
+    case 2: self = .received
+    case let code: self = .unknown(code: code)
+    }
+  }
+}