Add Tags to Custom Assets

The ability to apply Tags to Custom Assets, which can then flow into Dynamic Quantity invoicing. This would allow use cases like:
- a Firewall Asset that is billed as a managed device, and can also have a second tag related to the type of licence it has,
- a Linux server or NAS, perhaps with a tag representing a certain software licence or rental charge,
- any other items representing a managed device that needs billing but can’t run the Gorelo Agent directly.