Implement remote_servers fragmentation via ConfigMaps

[shankeleven]

Partition remote_servers into ConfigMap fragments

Motivation

Large ClickHouse clusters can generate remote_servers.xml exceeding Kubernetes ConfigMap size limits (~1 MiB), causing reconcile failures.
Solves #1874

What this PR does

• Splits remote_servers.xml into deterministic fragments when size exceeds threshold

• Adds configuration:

  clickhouse:
    configuration:
      remoteServers:
        remoteServersSplitThresholdBytes: 614400  # 600 KiB
        maxRemoteServersFragments: 100

• Preserves legacy behavior when size is below threshold

• Uses projected volume to mount fragments transparently

• Adds hash annotation to trigger deterministic pod rollout

• Implements safe GC:

  • post-StatefulSet reconcile
  • on CHI deletion

---

Testing

• Unit tests for fragmentation logic and edge cases

• E2E tests for:

  • fragment creation & mounting
  • GC after scale-down
  • max fragment failure scenario

---

Result

• This removes ConfigMap-size-related reconciliation risk for large deployments while keeping behavior unchanged for clusters that do not require fragmentation.

[shankeleven]

@sunsingerus @alex-zaitsev could you please review this PR