Add a `cancellationToken` for `WebApplication.RunAsync`

## Background and Motivation

Currently, the `WebApplication.RunAsync` does not have a `CancellationToken` parameter, think we should have a `CancellationToken` to forward it to the inner `Host`.

Similar issue: https://github.com/dotnet/aspnetcore/issues/44083

## Proposed API

```diff
namespace Microsoft.AspNetCore.Builder;

public sealed class WebApplication
{
public Task RunAsync([StringSyntax("Uri")] string? url = null);
+ public Task RunAsync([StringSyntax("Uri")] string url, CancenllationToken cancellationToken);
}
```

## Usage Examples

```csharp
using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(5));
var builder = WebApplication.CreateSlimBuilder();
var app = builder.Build();
await app.RunAsync("http://localhost:5000", cts.Token);
```

## Alternative Designs

``` diff
- public Task RunAsync([StringSyntax("Uri")] string? url = null);
+ public Task RunAsync([StringSyntax("Uri")] string? url = null, CancenllationToken cancellationToken = default);
```

## Risks