feat(dcl): extend CreateConnectionCommand with backup config and failover retry count

Update CreateConnectionCommand to carry PrimaryConnectionDetails,
BackupConnectionDetails, and FailoverRetryCount. Update all callers:
DataConnectionManagerActor, DataConnectionActor, DeploymentManagerActor,
FlatteningService, and ConnectionConfig. The actor stores both configs
but continues using primary only — failover logic comes in Task 3.

src/ScadaLink.Commons/Types/Flattening/FlattenedConfiguration.cs

@@ -33,6 +33,8 @@ public sealed record ConnectionConfig
 {
     public string Protocol { get; init; } = string.Empty;
     public string? ConfigurationJson { get; init; }
+    public string? BackupConfigurationJson { get; init; }
+    public int FailoverRetryCount { get; init; } = 3;
 }
 
 /// <summary>