Proposed additions to GrainCancellationTokenSource

It's good practice to make grain methods cancelable by passing a `GrainCancellationToken` to each grain method. However, if we are calling a grain method via an Orleans client, then we have a `CancellationToken`, not a `GrainCancellationToken`.

Ideally, grain methods would just accept a `CancellationToken` rather than a `GrainCancellationToken`, which would make grains more "POCO". However, in the absence of that, either or both of the following two additions would be helpful:
1. Add a `GrainCancellationTokenSource.CreateLinkedTokenSource(CancellationToken)` method like what exists on the `CancellationTokenSource` class. This would allow us to more easily create a `GrainCancellationTokenSource` linked to an existing `CancellationToken`).
2. Add an implicit converter from a `CancellationToken` to a `GrainCancellationToken` so we can pass an existing `CancellationToken` directly to a grain method accepting a `GrainCancellationToken`.

In the absence of either of the above, we are forced to write code like below.

```csharp
using var cts = new GrainCancellationTokenSource();
using (context.CancellationToken.Register(
cts => ((GrainCancellationTokenSource)cts!).Cancel(), cts))
{
await grain.Method(..., cts.Token);
}
```

Start with GrainCancellationTokenSource and its Token and Cancel usage, then compare the requested overload and conversion with CancellationTokenSource behavior. Review the existing 11-comment discussion before choosing whether to pursue either proposal; done means a client CancellationToken can reach a grain method without the manual registration shown.

Written by the indexing model from the issue text.