feat: rename validation builder for parity

Rename extensions functions and `IValidatorBuilder` for parity with
dispatcher options and builder.

src/request.validation/DispatchingValidator.cs

@@ -54,7 +54,7 @@ internal sealed class DispatchingValidator : IValidator
 	{
 		public override Validation Validate(ValidationContext context, IServiceProvider serviceProvider)
 		{
-			var options = serviceProvider.GetRequiredService<IOptions<ValidationOptions>>().Value;
+			var options = serviceProvider.GetRequiredService<IOptions<RequestValidatorOptions>>().Value;
 
 			var validators = options.GetValidators<T>(serviceProvider);