Add McEliece classic encryption

[zero42-042]

Hi XTLS team,

I would like to suggest considering experimental support for Classic McEliece in Xray core as an additional post-quantum option alongside existing lattice-based schemes like ML-KEM and ML-DSA.

While lattice-based cryptography is currently the practical standard, McEliece could provide a conservative, long-term security alternative for use cases where performance trade-offs are acceptable in exchange for maximum cryptographic robustness.

Best regards.