[Feature Request] Access to workflow/activity instance from context

[cretz]

Describe the solution you'd like

The created instance of the workflow class should be made available as Workflow.Instance static property.

Also, in Temporalio.Extensions.Hosting where we create and control the lifetime of the activity instance, the instance should be made available in ActivityExecutionContext.Instance instance property. This is probably best as an extension method on that class and it is understood to only be as available as the execution context itself is (i.e. async local, may not traverse threads).

See temporalio/features#572.