Add option to generate async-over-sync virtual methods for their sync counterparts

```csharp
class A
{
  public abstract void SomeMethod();
}
```
=>
```csharp
partial class A
{
  // Pseudo code. All rules shall apply
  public virtual async Task SomeMethodAsync()
  {
    SomeMethod();
  }
}
```
Possible controlled by an option, but I would prefer this to be a default behavior.