`ClientRequestAdapter.createPending` (and other methods?) should be optionally async

[brettimus]

When you create a pending server-to-client request, you may need to record information about that request in a persistence layer, like a KV store. These writes are asynchronous, meaning that the create action itself is async. The adapter should reflect that in its signature.

Example: You write information to an external KV store about the request you made to the client. As you await the response, you check the status of that record to see if a corresponding response has been received. that initial write to the KV store was async.