feat(keeper): add latency-based multi-RPC routing

[Tobiloba0]

Closes #817

Summary

• route keeper RPC calls to the highest-scoring healthy Stellar RPC endpoint
• score endpoints using latency, error rate, consecutive failures, and ledger-height lag
• add failover config knobs, docs, and tests for fastest-node and stale-node routing

Testing

• npm test -- --runTestsByPath tests/disasterRecovery.test.js --coverage=false
• npx eslint src/disasterRecovery.js src/config.js ../keeper/index.js tests/disasterRecovery.test.js

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[ayomideadeniran]

pr under review, i will get back to you if i find any wrong implementations