feat: add OP Stack integration tests and deploy RPC gateway

- Add opstack_integration_test.go for testing remote signer with OP Stack
- Add test-opstack.sh for quick curl-based verification
- Deploy RPC gateway with sign-evm EIP-155 support
- Add plugin/.gitignore to exclude binary artifacts
- Configure ingress for rpc.popsigner.com with TLS

The RPC gateway now supports:
- eth_accounts: List available signing addresses
- eth_signTransaction: Sign transactions (legacy + EIP-1559)
- eth_sign / personal_sign: Sign arbitrary messages
- API key authentication via X-API-Key header
- mTLS authentication for Arbitrum Nitro (port 8546)

Author: Bidon15