feat(node/rpc): implement admin state persistence

[Farukest]

Summary

Implements the --rpc.admin-state CLI flag functionality that was defined but never implemented.

When a file path is provided via --rpc.admin-state, the node will:

1. Load persisted admin state on startup (if file exists)
2. Save state changes whenever admin API modifies sequencer state

Changes

• Add AdminState struct: Serializable struct containing sequencer_active and recovery_mode fields
• Add AdminStatePersistence helper: Provides load() and save() methods for JSON file persistence
• Update SequencerActor: Add admin_state_persistence field to handle persistence
• Persist on state changes: Save state in start_sequencer, stop_sequencer, and set_recovery_mode
• Load on startup: Read persisted state in node.rs when initializing the sequencer

Persisted State

{
  "sequencer_active": true,
  "recovery_mode": false
}

Test plan

• Unit tests for AdminState serialization
• Unit tests for AdminStatePersistence (load/save/disabled scenarios)
• All existing tests pass (108/108)

Closes #3124

[wiz-b4c72f16a4]

Wiz Scan Summary

Scanner	Findings
Vulnerabilities	4
Sensitive Data	-
Secrets	-
IaC Misconfigurations	-
SAST Findings	-
Software Supply Chain Findings	-
Total	4

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