Add built-in support for ElGamal encryption

[kripod]

ElGamal is one of the available partial, but performant homomorphic encryption mechanisms. I started to write a separate library for it, but thought that the functionality I wish to provide could be fit into the official crypto API of Node.

Most of the implementation could be inherited from the available Diffie-Hellman methods, especially, key generation.

[addaleax]

I think node’s crypto module is pretty much going to stick with offering what OpenSSL has, but @nodejs/crypto folks can probably give better answers than I can.

[indutny]

I agree with @addaleax on this. Unfortunately, this is not provided as an API primitive for OpenSSL, so there is no reason for us to expose it as a part of the core. Nevertheless, this could be implemented as a user-land C++ addon.