Add RemoteSlurmJob to connect SLURMCluster to a remote Slurm cluster

Hello 👋 Thank you for considering this feature request :) I have been looking over dask-jobqueue (together with prefect) to allocate resources on a Slurm cluster I have access to. dask-jobqueue seems exactly what we'd need for this, thank you for maintaining it 🙇

### Context

In the case where Slurm and a python process (script or notebook) are not running on the same host SlurmCluster will not be able to spawn any jobs and error.

Slurm added a REST API: https://slurm.schedmd.com/rest_api.html

### Feature

Could we add a `RemoteSlurmCluster` and `RemoteSlurmJob` that largely extend SlurmCluster/SlurmJob and instead of using `subprocess.Popen` we'd do an HTTP request instead?

```
class RemoteSLURMJob(SLURMJob):
@contextmanager
def job_file(self):
# we don't need the script file but only the script itself. to not alter `async def start(self):` we yield the script here.
yield self.job_script()

async def _submit_job(self, script):
# formatting should be according to:
# https://slurm.schedmd.com/rest_api.html#slurmctldSubmitJob
return requests.post('slurm-url/jobs/submit') # reach out to API

def _job_id_from_submit_output(self, out):
# out is the JSON output from _submit_job request.post
# See https://slurm.schedmd.com/rest_api.html#v0.0.36_job_submission_response
return out['job_id']

@classmethod
def _close_job(cls, job_id):
# See: https://slurm.schedmd.com/rest_api.html#slurmctldCancelJob
requests.delete(f'slurm-url/job/{job_id}')

class RemoteSLURMCluster(SLURMCluster):
job_cls = RemoteSLURMJob
```

As far as I can tell this should be a drop-in replacement.

Thoughts? (tagging @mrocklin @lesteve for visibility, hope you would have time for a review.)