Refactor NewWorker and remove downcast

[alexshtin]

#69 added downcast and panic to NewWorker in internal/worker.go. This needs to be refactored and removed.

[jinhong-]

I am facing an issue with this at the moment when i attempt to wrap temporal client with my own implementation. Will this eventually be removed, and what is the reason for this explicit check?

[cretz]

As part of the new interceptors proposal at https://github.com/temporalio/proposals/pull/45 client interceptors are supported to allow interception. However, due to the worker still needing internal things from the client, it can't be wrapped, but all functionality should be available via interceptors.