[Multi-Tenant] Add tenantId to assets

[njlie]

Because assets aren't associated with a Wallet Address like Open Payments resources are, it becomes necessary to add a tenantId to them to determine which tenant that asset belongs to.

• Require a tenantId to be passed into the assetService.create call
• Filter any getters on the asset service by tenantId, unless the requester is the operator of the Rafiki instance