This PR adds two additional constraints on Connect sidecar and gateway tasks,
making sure Nomad schedules them only onto nodes where Connect is actually
enabled on the Consul agent.

Consul requires `connect.enabled = true` and `ports.grpc = <number>` to be
explicitly set on agent configuration before Connect APIs will work. Until
now, Nomad would only validate a minimum version of Consul, which would cause
confusion for users who try to run Connect tasks on nodes where Consul is not
yet sufficiently configured. These contstraints prevent job scheduling on nodes
where Connect is not actually use-able.

Closes #10700