at_c: atchops_rsa_generate

[JeremyTubongbanua]

Implement this function in atchops - rsa.c to generate an RSA 2048 public/private keypair

This function returns int for error handling; 0 for success, non-zero implies failure

As to how to do this: one hint is to explore and use MbedTLS and its random/entropy libraries to potentially do this.

int atchops_rsa_generate(atchops_rsakey_publickey *publickey, atchops_rsakey_privatekey *privatekey, const unsigned int keysize)
{
    return 1; // TODO: implement
}

Also write tests