Update ActivityKind values

Originally taken from #692 ([comment](https://github.com/Azure/durabletask/pull/692#discussion_r836667217)).

> It would be good to also set ActivityKind to an appropriate value based on the activity at had:
>
> TaskHubClient -> Producer (this produces a new message on the DTFx queue)
> TaskHubWorker processing an orchestration or activity -> Server (this is the 'server' running work)
> TaskHubWorker sending an outbound orchestration or activity -> Client (this is acting as a 'client' waiting for external work to process)
>
> dtfx.type should then be used to indicate what is the task type, as in it is only even Activity or Orchestration values. So it can be read as "this ActivityKind.Server span is processing a dtfx.type of Orchestration"