Add SSSD support to controller

[faganihajizada]

Summary

Adds optional SSSD support to the controller component, enabling user validation for Slurm reservations with enterprise LDAP/Active Directory accounts.

Problem: Users cannot create reservations with specific user accounts when those users don't exist locally in the controller pod:

$ scontrol create reservationname=test-res starttime=now duration=100 nodes=node-1 users=alice
Error creating the reservation: Unspecified error

Root Cause: Controller (slurmctld) validates user accounts when creating reservations, but only local users are available without SSSD integration. LoginSet and NodeSet already support SSSD, but Controller did not.

Breaking Changes

• Security Context Change
• Controller pod now runs as root (required for SSSD)

Testing Notes

Unit tests:

$ go test -v ./internal/builder -run TestBuilder_BuildController
=== RUN   TestBuilder_BuildController
--- PASS: TestBuilder_BuildController (0.03s)
PASS
ok      github.com/SlinkyProject/slurm-operator/internal/builder        0.600s

Verification:

$ kubectl get pod cluster1-controller-0 -n slurm-cluster1 -o json | jq '.spec.securityContext'
{"fsGroup": 401}

$ kubectl exec cluster1-controller-0 -c slurmctld -- ps aux | grep -E "sssd|slurmctld"
root   16  sssd -i                      
root   47  sssd_be --domain DEFAULT     
root   48  sssd_nss                     
root   49  sssd_pam                    
slurm  59  slurmctld --systemd   

Additional Context

Provided in support ticket with explored approaches