Create Durable Agent Management APIs

[alicejgibbons]

Just like the Workflow management APIs exist for Dapr workflow , the associated APIs should exist for Durable workflows.

Today you can get the status and start a durable workflow but terminating, purging, raising an event, and pause/resume would be useful. You can of course manually do terminate by killing the agent in various ways but doing this circumvents the agent itself and isn't best practice. There should be a mechanism by which the user can tell the agent to terminate the workflow, etc.

[CasperGN]

This work has already been started in #311 - we need to first handle #316 (I'm on it atm) and then #312