Ongoing work to support Nomad Workload Identity for authenticating with Consul
will mean that Nomad's service registration sync with Consul will want to use
Consul tokens scoped to individual workloads for registering services and
checks. The `ServiceRegisterOpts` type in the API doesn't have an option to pass
the token in, which prevent us from sharing the same Consul connection for all
workloads. Add a `Token` field to match the behavior of `ServiceDeregisterOpts`.
Tim Gross