Workflow: no longer require DB connection string

[JoshVanL]

PR updates the workflow clients to use the Dapr RPCs to list Workflow instance IDs instead of connecting directly to the database. This removes the requirement that the Dapr CLI need both connectivity to the database and a connection string, greatly improving the user experience.

$ dapr workflow list now works out-of-the-box without any additional configuration.

Connection strings are still supported via the --connection-string flag for users using Dapr pre v1.17.

Also exposes the --force flag on $ dapr workflow purge to allow purging without a worker connected.