[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

Tests/Persistence/Extensions/URL+DevicesTests.swift

@@ -0,0 +1,30 @@
+//
+//  URL+DevicesTests.swift
+//  PersistenceTests
+//
+//  Created by Javier Cicchelli on 17/04/2023.
+//  Copyright © 2023 Röck+Cöde. All rights reserved.
+//
+
+import Foundation
+import Persistence
+import XCTest
+
+final class URL_DevicesTests: XCTestCase {
+    
+    // MARK: Properties
+    
+    private var url: URL!
+    
+    // MARK: - Tests
+    
+    func test_bitBucket() {
+        // GIVEN
+        // WHEN
+        url = .bitBucket
+        
+        // THEN
+        XCTAssertEqual(url.absoluteString, "file:///dev/null")
+    }
+    
+}