Allow listing parent/children jobs only in API

In hashi-ui and other tasks i've encountered where I need to get a list of jobs and parse them to validate certain things, I always end up doing stuff similar to the code below

It would be nice to allow richer filtering on the job list, e.g. only "root" jobs (`*job.ParentID == nil`) or jobs with a specific meta key set

In prioritised order

- [ ] allow only showing parent jobs
- [ ] allow only showing children jobs
- [ ] allow filtering on `meta{}` kv in each job

```go
jobs, _, err := client.Jobs().List(&api.QueryOptions{Prefix: appName + "-", AllowStale: true})
if err != nil {
log.Fatalf("Could not list remote jobs: %s", err)
}

found := make([]string, 0)
for _, jobStub := range jobs {
if strings.Contains(jobStub.ID, "/periodic-") {
log.Debugf("Job %s are a periodic child job, ignoring")
continue
}

job, _, err := client.Jobs().Info(jobStub.ID, nil)
if err != nil {
log.Fatalf("Could not read remote job %s: %s", jobStub.ID, err)
}

// This is periodic and paramertized jobs
// we don't want to interfere with those auto-created jobs as they will be gc'ed by
// nomad on their successful completelion
if *job.ParentID != "" {
log.Debugf("Job %s is child of %s, skipping", *job.ID, *job.ParentID)
continue
}

found = append(found, *job.ID)
}
```

Start with the client.Jobs().List entry point and its api.QueryOptions, then trace how job lists are handled by the API. The requested behavior includes parent-only, child-only, and metadata key/value filtering; done means callers can select these filters without fetching each job individually.

Written by the indexing model from the issue text.