Scaffold max external service enforcement on a per-tier basis
Created by: flying-robot
This introduces two things:
- An external service count enforcement that will be predicated on the current license tier
- A pre-create hook in the external services store to execute that enforcement
A subsequent PR between myself and @unknwon will wrap the licensing Info
type with values (e.g. max external services etc.) that we can use where the current TODO in external_services.go is present.