Added input validation to the Authentication middleware (#5)

This PR contains the work done to improve the existing `AuthMiddleware` type to provide input validations with the `SecureValidationRule` validation rule and also, by generating the authentication information at initialization time.

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

Sources/DiscogsService/Internal/Validation Rules/NotEmptyValidationRule.swift

@@ -29,6 +29,17 @@ struct NotEmptyValidationRule: InputValidationRule {
     
 }
 
+// MARK: - Definitions
+
+extension InputValidationRule where Self == NotEmptyValidationRule {
+    
+    // MARK: Constants
+    
+    /// A validation rule that checks whether an input is empty or not.
+    static var notEmpty: Self { .init() }
+    
+}
+
 // MARK: - Helpers
 
 private extension NotEmptyValidationRule {
@@ -54,10 +65,3 @@ private extension NotEmptyValidationRule {
     }
     
 }
-
-// MARK: - Constants
-
-extension InputValidationRule where Self == NotEmptyValidationRule {
-    /// A validation rule that checks whether an input is empty or not.
-    static var notEmpty: Self { .init() }
-}