Support AddAsKeyed on AddGrpcClient

### Is your feature request related to a problem? Please describe.
I want to used Keyed Services feature with named grpc clients. Right now i have something like this:

```csharp
serviceCollection.AddGrpcClient("custom_name", o =>
{
o.Address = new Uri(someUrl);
});
```

But to access this named client i have to inject `GrpcClientFactory` and all `Create` on it. In .NET 9 `IHttpClientBuilder` introduced `AddAsKeyed` that does exactly that for http clients, but it seems to be ignored for grpc clients.

### Describe the solution you'd like
What i would love to have is that i can inject my named client directly like that:
```csharp
app.MapGet("/", ([FromKeyedServices("custom_name")] MyGrpcServiceClient client) => ... );
```

Start by reading the AddGrpcClient registration path and comparing it with .NET 9's IHttpClientBuilder.AddAsKeyed behavior. Confirm the change should register named gRPC clients for [FromKeyedServices("custom_name")] injection, with direct keyed resolution replacing the need for GrpcClientFactory.Create.

Written by the indexing model from the issue text.