[Feature] Persistence (#5)

This PR contains the work done to implement some useful protocols, classes and extensions to use to setup a persistence layer in an application.

To provide further details about the work done:
 - [x] declared the `Persistence` target in the Package file;
 - [x] forgot to declare the `Communications` and `Persistence` target to the `SwiftLibs` library in the `Package` file;
 - [x] defined the `Service` public protocol;
 - [x] implemented the `Fetcher` generic class;
 - [x] implemented the `bitBucket` static property in the `URL+Devices` public extension;
 - [x] updated the `README` file.

Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Reviewed-on: #5

Sources/Persistence/Extensions/URL+Devices.swift

@@ -0,0 +1,18 @@
+//
+//  URL+Devices.swift
+//  Persistence
+//
+//  Created by Javier Cicchelli on 17/04/2023.
+//  Copyright © 2023 Röck+Cöde. All rights reserved.
+//
+
+import Foundation
+
+public extension URL {
+    
+    // MARK: Properties
+    
+    /// URL to a virtual device in which any data written vanishes or disappear.
+    static let bitBucket = URL(fileURLWithPath: "/dev/null")
+
+}