[FEAT] support multiple endpoints

[FEAT] [SRV] addGroup() to group endpoints under a subject token
[CHANGE] [SRV] endpoint is now optional
[CHANGE] [SRV] service info now reports multiple subjects - one per endpoint, so `subject` change to `subjects` and is an array.
[CHANGE] [SRV] stats now have `endpoints` which is an array of endpoint stats.
[CHANGE] [SRV] ServiceStats no longer contain metric information at the root level, they are nested under endpoints as the service can now have one or more endpoints.

[CHANGE] addGroup can have contain subject tokens, but addEndpoint(name) must be a simple string - name can be {name: string, subject: string} to allow the service have a simple name but specify a complex subject