Add serialize/deserialize to ElGamalPubkey and AeCiphertext

[nikhil-helius]

Problem

Allow serialisation derivatives for AeCiphertextVisitor , ElGamalCiphertext and ElGamalPubkey in order to support token-22 extensions serialisation. See https://github.com/solana-labs/solana-program-library/pull/5437

[buffalojoec]

Hey @joncinque are we duplicating work here, or is this necessary?

https://github.com/solana-labs/solana-program-library/blob/acabef3680e972baf8e9a47531c35eaa45405740/token/program-2022/src/serialization.rs#L112-L147

[samkim-crypto]

Yes, I think it makes more sense to add serialization in zk-token-sdk. We can eventually remove the ones in token-2022.

[ghost]

Sorry for the delay. Got busy with a lot of other things. Planning to work on this, this weekend