[FEATURE] Add ability to query group memberships of a node

[synackd]

Is your feature request related to a problem? Please describe.
Not a problem, but a lack of functionality.

Describe the solution you'd like
It's currently possible to view the list of nodes for a group, but it would be useful to go the reverse and view the groups that a node is part of.

The SMD API already supports this, so it should be simple to implement:

$ curl -s -H "Authorization: Bearer $DEMO_ACCESS_TOKEN" https://demo.openchami.cluster:8443/hsm/v2/memberships/x1000c0s0b0n0 | jq
{
  "id": "x1000c0s0b0n0",
  "groupLabels": [
    "all",
    "compute",
    "slurm"
  ],
  "partitionName": ""
}

Describe alternatives you've considered
Not having this, but having it has been suggested multiple times.

Additional context
N/A