feature(#6): this commit introduces the implementation of the permissions namespace policy service

Author: https-richardy

Source/HttpsRichardy.Federation.Application/Policies/PermissionNamespacePolicy.cs

@@ -0,0 +1,15 @@
+namespace HttpsRichardy.Federation.Application.Policies;
+
+public sealed class PermissionNamespacePolicy : IPermissionNamespacePolicy
+{
+    public async Task<Result> EnsurePermissionIsAllowedAsync(
+        Realm realm, Permission permission, CancellationToken cancellation = default)
+    {
+        var isReserved = RealmPermissions.SystemPermissions
+            .Contains(permission.Name);
+
+        return isReserved
+            ? Result.Failure(PermissionErrors.PermissionNameIsReserved)
+            : Result.Success();
+    }
+}