Add optional/ephemeral volume notation for restore resilience

Introduce x-optional-volumes for beacon sidecars (prysm, nimbus, lighthouse,
teku, lodestar) that are checkpoint-sync capable and rebuild state quickly.
Mark reth sockets volumes as ephemeral (runtime IPC artifacts).

Script changes:
- volume-utils.sh: add is_optional_volume_key() function
- restore-volumes.sh: tolerate missing optional/ephemeral volume archives
- clone-backup.sh: skip missing optional/ephemeral volumes gracefully
- list-restorable.sh: only require archives for non-optional persistent volumes

This fixes the de-25 ethereum prysm incident (entire 1.1T restore failed
due to missing prysm archive) and arc-testnet reth sockets restore loops.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

gnosis/nethermind/gnosis-chiado-nethermind-archive-rocksdb-trace.yml

@@ -181,4 +181,5 @@ x-upstreams:
       generic:
         rpc:
           url: $${RPC_URL}
+x-optional-volumes: [gnosis-chiado-nethermind-archive-rocksdb-trace_nimbus]
 ...