This nonce generator uses an RNG to generate nonces. The RNG quality is
currently set to RNG_WEAK which is the same value used in IKE init.
The plugin is enabled and thus built by default.
Nonce generators (nonce_gen_t) can be used to get or allocate nonces.
Users can request nonce generators from the crypto factory while nonce
plugins register/remove themselves to/from the crypto factory.