Add AuthExtensions with AuthorizePolicyAsync method

Introduced a static AuthExtensions class providing an AuthorizePolicyAsync extension method for IAuthorizationService. This method streamlines policy-based authorization checks by returning a boolean result for a given user and policy name.

EnvelopeGenerator.API/Extensions/AuthExtensions.cs

@@ -0,0 +1,25 @@
+using Microsoft.AspNetCore.Authorization;
+using System.Security.Claims;
+
+namespace EnvelopeGenerator.API.Extensions;
+
+/// <summary>
+/// 
+/// </summary>
+public static class AuthExtensions
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="authService"></param>
+    /// <param name="user"></param>
+    /// <param name="policyName"></param>
+    /// <returns></returns>
+    public static async Task<bool> AuthorizePolicyAsync(this IAuthorizationService authService,
+                                                  ClaimsPrincipal user,
+                                                  string policyName)
+    {
+        var result = await authService.AuthorizeAsync(user, policyName);
+        return result.Succeeded;
+    }
+}