Backend: Create an "Add Device" endpoint

A new mutation should be added to the BFF service to support the frontend work in https://issues.opennms.org/browse/HS-194

• The mutation should be a simple "add device" that is tied directly to the button in the frontend
• You can currently add a device in Core by creating a requisition with a node in it.
• The BFF service is intended to hide backend complexity from the frontend. The resolver for the mutation should create a requisition containing the new node.

Out of scope:

• Device and inventory management by deleting/modifying devices
• SNMP agent management, such as communication ports, and SNMP agent version v1/v2c/v3