Add Polly resilience policies

clients/dotnet/MxGateway.Client/MxGatewayClientOptions.cs

@@ -21,6 +21,8 @@ public sealed class MxGatewayClientOptions
 
     public TimeSpan DefaultCallTimeout { get; init; } = TimeSpan.FromSeconds(30);
 
+    public MxGatewayClientRetryOptions Retry { get; init; } = new();
+
     public ILoggerFactory? LoggerFactory { get; init; }
 
     public void Validate()
@@ -54,5 +56,7 @@ public sealed class MxGatewayClientOptions
                 nameof(DefaultCallTimeout),
                 "The default call timeout must be greater than zero.");
         }
+
+        Retry.Validate();
     }
 }