Add support for default values

[boma96]

Specifying default values which are embedded in workflow definition is now supported

public class DefaultValueWorkflowInput : WorkflowInput<DefaultValueWorkflowOutput>
{
    [DefaultValue("Default name")]
    public string Name { get; set; }

    [DefaultValue("Default address")]
    public string Address { get; set; }
}

public class DefaultValueWorkflowOutput : WorkflowOutput { }

public class DefaultValueWorkflow : Workflow<DefaultValueWorkflow, DefaultValueWorkflowInput, DefaultValueWorkflowOutput>
{
    public EmailPrepareV1 PrepareEmail { get; set; }

    public DefaultValueWorkflow(WorkflowDefinitionBuilder<DefaultValueWorkflow, DefaultValueWorkflowInput, DefaultValueWorkflowOutput> builder)
        : base(builder) { }

    public override void BuildDefinition()
    {
        _builder.AddTask(wf => wf.PrepareEmail, wf => new() { Name = wf.WorkflowInput.Name, Address = wf.WorkflowInput.Address });
    }
}