Add safe-prime generator and checker to paillier.c

[nmajid2]

Goal

To integrate safe-prime generation with Paillier , we can add a safe-prime generation function plus a safe-prime checker to paillier.c. Thus, whenever an RNG instance is provided to PAILLIER_KEY_PAIR, it can call the safe prime generator. Moreover, the correctness of given primes (either given as input of PAILLIER_KEY_PAIR or from the safe-prime generator) can be checked using the safe-prime checker. We can also expose both new functions to be reached by the other codes.

Acceptance criteria

1. Well documented code
2. Tests pass with more than 80% test coverage
3. Static analysis should pass
4. Code should be reviewed by two engineers