[Feat Request] Globally configure Variable storage driver

## Feature Request

### Problem Overview

In cases where a (programmatic) workflow has many variables, it would be useful to globally configure the variable storage driver instead of specifying it for every variable.

### Proposed Solution

Instead of doing this:

```cs
var myVariable1 = workflowBuilder.WithVariable().WithMemoryStorage();
var myVariable2 = workflowBuilder.WithVariable().WithMemoryStorage();
var myVariable3 = workflowBuilder.WithVariable().WithMemoryStorage();
```

or

```cs
var myVariable1 = new Variable().WithMemoryStorage();
workflowBuilder.Variables.Add(myVariable1);

var myVariable2 = new Variable().WithMemoryStorage();
workflowBuilder.Variables.Add(myVariable2);

var myVariable3 = new Variable().WithMemoryStorage();
workflowBuilder.Variables.Add(myVariable3);
```

It would be useful to do something like

```cs
workflowBuilder.WorkflowOptions.StorageDriverType = typeof(MemoryStorageDriver);
```

Then we don't need to specify the storage driver when creating Variables eg:

```cs
var myVariable1 = workflowBuilder.WithVariable();
var myVariable2 = workflowBuilder.WithVariable();
var myVariable3 = workflowBuilder.WithVariable();
```

or

```cs
var myVariable1 = new Variable();
workflowBuilder.Variables.Add(myVariable1);

var myVariable2 = new Variable();
workflowBuilder.Variables.Add(myVariable2);

var myVariable3 = new Variable();
workflowBuilder.Variables.Add(myVariable3);
```

### Additional Context

https://discord.com/channels/814605913783795763/1385515616705708132/1386606615905435708