Replace point-to-point communication between deviceauth and inventory with workflows

When we released Mender 2.4, we were in a hurry as it was a last-minute discovery and decided to have direct communication between deviceauth and inventory to update the device identity, instead of using workflows, which decouples the two microservices and provide retries.

Replace the direct communication with a workflow.

References:

1. https://github.com/mendersoftware/deviceauth/blob/master/devauth/devauth.go#L224
2. https://github.com/mendersoftware/deviceauth/blob/master/devauth/devauth.go#L994