Serialization of kem::DecapsulationKey

[kc1212]

Problem:

For ML-KEM, the encapsulation key can be serializd using key_bytes but there are no similar functions for DecapsulationKey.

For many application, the decapsulation key must be stored on some persistent storage which is not possible with the current API if not mistaken.

Solution:

Add a method to serialize and deserialize a decapsulation key.

This is defined in the ByteEncode (Algorithm 5) function of the NIST spec https://nvlpubs.nist.gov/nistpubs/fips/nist.fips.203.pdf and used in, for example Algorithm 13 during key generation. I'm not sure if the openssl API already has it.

[justsmth]

Hello! Thanks for contact us about this.

We plan to support this feature in aws-lc-rs, but we first need to make a few changes upstream in AWS-LC. We will follow-up here as we make progress.

[justsmth]

Related: aws/aws-lc#2624