[🚀 Feature]: [dotnet] Small step forward to Async

### Description

Nobody knows when Async https://github.com/SeleniumHQ/selenium/issues/14067 will be in place, and moreover how to do it smoothly without pain for end-users.

I propose to make small improvement now, even before v5.

### Current
```csharp
var driver = new ChromeDriver();
```

### Proposal:
```csharp
var driver = await ChromeDriver.StartAsync();
```

### Have you considered any alternatives or workarounds?

In any case constructor cannot be async, we will rework it entirely. Ha, and it means everybody has to use another API.

I propose alternative approach here.

Bad?
```csharp
var driver = new ChromeDriver();
await driver.StartAsync();
```

Better:
```csharp
var driver = await ChromeDriver.StartAsync();
```

We can try to make it without breaking changes. Seems it is small step, but it contributes significantly. This is most tricky part of migration.