`AnyEncryptionKey`: expose optimized `pow_mod_n`

[survived]

Expose method AnyEncryptionKey::pow_mod_n(x, e) that returns $x^e \bmod N$ where $N$ is Paillier public key. It is optimized when factorization of $N$ is known.

It can be used, for instance, in paillier-zk crate.