Extended API for More Efficient Key Validation

[bhess]

Context: FIPS 203 introduces additional key validation during the encapsulation and decapsulation process. In the ML-KEM implementation from mlkem-native, this results in overhead of ~5% in encapsulation and ~30% in decapsulation.

Currently, with the 3-way liboqs KEM API (OQS_KEM_keypair, OQS_KEM_encaps, OQS_KEM_decaps) key validation checks need to be performed during each encapsulation and decapsulation operation, adding overhead for every operation.

An extended API that separates key import from encapsulation and decapsulation would allow the validation checks to occur only once during key import. This would eliminate the repeated overhead when performing multiple encapsulation or decapsulation operations on the same key.

This issue aims to track the investigation and potential development of such an extended API.