Feature-based Consul constraint

### Proposal

In Nomad v1.3.0 jobs that use Consul service discovery received [an implicit constraint for the Consul version](https://github.com/hashicorp/nomad/blob/v1.3.2/nomad/job_endpoint_hooks.go#L27-L35). But this constraint is hard-coded to 1.7.0, since this is version that added support for namespaces, but versions prior to that would work if the service doesn't make use of the namespace feature.

It would be better if the constraint value was based on the features being used, so, for example, if the service has a Consul namespece set, the constraint is 1.7.0, but without namespace, there would be no version requirement.

### Use-cases

Using Nomad with Consul versions prior to 1.7.0.

Start with nomad/job_endpoint_hooks.go, particularly the implicit Consul version constraint introduced for service discovery. Trace how service namespaces are represented in the job configuration and how the constraint is generated; done means the required version reflects whether namespace functionality is used while preserving compatibility for services without namespaces.

Written by the indexing model from the issue text.