Make multistream capability configurable

There's a todo item in the GrpcWorkerChannel to make `MultiStream` capability configurable, instead of hardcoded to `10`

```csharp
// advertise that we support multiple streams, and hint at a number; with this flag, we allow
// clients to connect multiple back-hauls *with the same workerid*, and rely on the internal
// plumbing to make sure we don't process everything N times
initRequest.Capabilities.Add(RpcWorkerConstants.MultiStream, "10"); // TODO: make this configurable
```

[Code](https://github.com/Azure/azure-functions-host/blob/a716dd28ac7a2dfa5542e0b8a5b7a5a584683b4d/src/WebJobs.Script.Grpc/Channel/GrpcWorkerChannel.cs#L336)

Start in src/WebJobs.Script.Grpc/Channel/GrpcWorkerChannel.cs around the TODO at line 336 and trace how the MultiStream capability is advertised. Identify the existing configuration path for worker-channel settings; done means the advertised value is configurable rather than hardcoded to 10, with coverage for the resulting capability value.

Written by the indexing model from the issue text.