feature(#20): this commit introduces an audience association schema validator for clients

Author: https-richardy

Applications/Backend/Source/HttpsRichardy.Federation.Application/Validators/Client/AssignClientAudienceValidator.cs

@@ -0,0 +1,13 @@
+namespace HttpsRichardy.Federation.Application.Validators.Client;
+
+public sealed class AssignClientAudienceValidator : AbstractValidator<AssignClientAudienceScheme>
+{
+    public AssignClientAudienceValidator()
+    {
+        RuleFor(scheme => scheme.Value)
+            .NotEmpty()
+            .WithMessage("audience value is required.")
+            .MaximumLength(500)
+            .WithMessage("audience value cannot exceed 500 characters.");
+    }
+}