[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/Helpers/NSFetchRequest+TestEntity.swift

@@ -0,0 +1,24 @@
+//
+//  NSFetchRequest+TestEntity.swift
+//  PersistenceTests
+//
+//  Created by Javier Cicchelli on 17/04/2023.
+//  Copyright © 2023 Röck+Cöde. All rights reserved.
+//
+
+import CoreData
+
+extension NSFetchRequest where ResultType == TestEntity {
+    
+    // MARK: Functions
+    
+    static func allTestEntities() -> NSFetchRequest<TestEntity> {
+        let request = TestEntity.fetchRequest()
+        
+        request.sortDescriptors = []
+        request.resultType = .managedObjectResultType
+        
+        return request
+    }
+
+}