[API Proposal]: FakeTimeProvider.Advance

### Background and motivation

Currently, FakeTimeProvider.Advance(); returns a void.

When called, it runs all callbacks, for instance continuations of a PeriodicTimer.WaitForNextTickAsync();
However, as soon as these callback hits an await keyword, the thread jumps to the next waiter.
The rest of the calblacks eventually execute, but there is no reliable way to wait for their completion.
I think these waiters should be returned as a Task.WhenAll() awaitable from Advance();

### API Proposal

```csharp
namespace Microsoft.Extensions.Time.Testing;

public class FakeTimeProvider : TimeProvider
{
public Task Advance(TimeSpan delta);
}
```

### API Usage

```csharp
[Fact]
public async Task GivenCreatedTransaction_StatusIsAbortedAfterTimeout()
{
using var client = application.factory.CreateClient().WithAuthenticationAsParkingService();
var transaction = await client.CreateTransaction().AsTransactionDto();

await application.TimeProvider.Advance(application.PaymentOptions.TransactionLifetime!.Value);

var actual = await client!.GetTransaction(transaction!.Guid).AsTransactionDto();
Assert.Equal(TransactionState.Aborted.ToString(), actual!.State);
}
```

### Alternative Designs

_No response_

### Risks

_No response_