Add support to clear Service Bus session state

### Description

Service Bus session state support was added in #2102, but clearing the state is not yet supported.

As mentioned there as well, the Azure Service Bus SDK and the previous functions in-process SDK both support using `SetSessionStateAsync(null)` (or equivalent). The key is using `null` to clear the state, which is not possible with the current isolated SDK.

Can this be added to the isolated SDK?
There's no full support for session state management until clearing is possible (there will be an ever increasing amount of dangling session states). Workarounds to try setting the state via the Azure Service Bus client SDK models itself is also not possible, due to the state locking mechanism inside the client.