Interface for counting tokens

Most providers now have an API for counting tokens, i.e. submit the full request as you otherwise would, but get back a token count rather than an actual response. We should consider adding an interface for this purpose, e.g.
```csharp
Task CountTokensAsync(IEnumerable, ChatOptions, CancellationToken);
```
It could be implemented by IChatClients that support it and served up from their GetService implementations.