feat: add aztec sequencer integration

[itenev]

Summary

This PR introduces support for running an Aztec Sequencer node within the eth-docker stack.

> Note: Comprehensive setup and key migration documentation has been added to the eth-docker-docs repository. See companion PR: https://github.com/ethstaker/eth-docker-docs/pull/584

Key Changes

- aztec-sequencer.yml: A new modular compose file that integrates the sequencer with the existing execution, consensus, and web3signer services. It includes native Grafana Alloy auto-discovery labels (metrics.scrape=true, logs.collect=true).
- default.env: Added default AZTEC_* environment variables for operator configuration.
- Keystore Template: Added aztec/keystore.sample.json demonstrating the hybrid key storage approach required by Aztec:
  - 42-character ETH/publisher addresses are delegated to web3signer.
  - 66-character BLS private keys are stored inline (as Aztec's BLS requirements are incompatible with Web3Signer's consensus-layer implementation).
- .gitignore: Added /aztec/keys/keystore.json to prevent accidental key commits.

Operator Checklist

Before enabling this service, operators should ensure:
1. web3signer, execution, and consensus are active and healthy.
2. web3signer.yml,aztec-sequencer.yml is appended to the COMPOSE_FILE variable in .env.
3. AZTEC_P2P_IP is set to the node's public IP address for P2P discovery.
4. A valid keystore.json (generated via the Aztec CLI) is placed in aztec/keys/.
5. The sequencer identity has the required 200,000 AZTEC tokens staked, and the publisher account holds ≥0.1 ETH for L1 gas.

Testing

- Validated compose syntax: docker-compose -f aztec-sequencer.yml config.
- Verified metrics endpoint is exposed and scraped by Grafana Alloy.