[Feature request] client-level constraints

There are many scenarios where one would like to ensure that a particular node only receives either a certain job(s) or task group(s). As I see it, there is no way of restricting nodes to particular job(s) or task group(s) without introducing constraints in some way on job definitions themselves (e.g. using `meta` or `node_class`). There are workarounds, for example restricting those jobs to use different drivers that are only used for those jobs, but that's more of a hacky workaround that will only work for certain cases.

It'd be great if the `client` stanza had some mechanism to whitelist certain jobs or task groups - be it by a `constraint` stanza, requiring certain `meta` fields, or even just a whitelist of job or task group names.

Start by reviewing how the Nomad client stanza, job definitions, task groups, meta, node_class, and existing constraints relate. Compare the proposed constraint, metadata, and job or task-group whitelist approaches, then define which client-level behavior should be supported and how it would be validated.

Written by the indexing model from the issue text.