Add mlkem768x25519 (to sunset?) and integrate into SSH-Stamp

[brainstorm]

This post-quantum Kex is increasingly popular both OpenSSH and Dropbear SSH have it already... attractive too on an "official" level?: https://www.theregister.com/2024/12/17/australia_dropping_crypto_keys/

This should be feature-gated (non-default?) to keep binary size under control.

Prior recent art on Espressif's ESP-IDF: wolfSSL/wolfssl#8813

[brainstorm]

mkj/sunset@ce41739

Thanks to @mkj

[brainstorm]

From earlier Matt's conversation/context:

Extra scope to claim this works well on no_std and present to NLNet (currently it uses 3KB of RAM):

1. Make use of generate_deterministic: only needs 32 byte seed stored in session state (careful about CPU cycles impact).
2. Modify ml-kem crate itself to avoid needing to hold the ecapsulation/decapsulation key simultaniously. That'd avoid using 1.5KB stack space.