[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/Protocols/Service.swift

@@ -0,0 +1,36 @@
+//
+//  Service.swift
+//  Persistence
+//
+//  Created by Javier Cicchelli on 13/04/2023.
+//  Copyright © 2023 Röck+Cöde. All rights reserved.
+//
+
+import CoreData
+
+public protocol Service {
+    
+    // MARK: Properties
+    
+    /// The main managed object context.
+    var viewContext: NSManagedObjectContext { get }
+    
+    // MARK: Functions
+    
+    /// Create a private queue context.
+    /// - Returns: A concurrent `NSManagedObjectContext` context instance ready to use.
+    func makeTaskContext() -> NSManagedObjectContext
+    
+    /// Create a child context of the view context.
+    /// - Returns: A generated child `NSManagedObjectContext` context instance ready to use.
+    func makeChildContext() -> NSManagedObjectContext
+    
+    /// Save a given context.
+    /// - Parameter context: A `NSManagedObjectContext` context instance to save.
+    func save(context: NSManagedObjectContext) throws
+    
+    /// Save a given child context as well as its respective parent context.
+    /// - Parameter context: A child `NSManagedObjectContext` context instance to save.
+    func save(childContext context: NSManagedObjectContext) throws 
+    
+}