Add serialize/deserialize to ElGamalPubkey and AeCiphertext

[ghost]

Problem

Allow serialisation derivatives for AeCiphertextVisitor , ElGamalCiphertext and ElGamalPubkey in order to support token-22 extensions serialisation. See solana-labs/solana-program-library#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

[joncinque]

There is some duplication of work, but if it's done at a lower level, then we can eventually remove the ones in token-2022. @samkim-crypto do you think it's ok to add these directly into the zk-token-sdk?

[joncinque]

Since serde is only imported when the target_os is not solana, this PR is creating downstream build errors when trying to build on-chain programs: https://github.com/solana-labs/solana/actions/runs/6567978881/job/17850122613?pr=33760

Can you wrap all of these in #[cfg(not(target_os = "solana"))] or #[cfg_attr(not(target_os = "solana"), derive(Serialize, Deserialize))]?

[joncinque]

Are you sure you want to serialize / deserialize with byte arrays? The serializers pointed out by Joe all use string representations https://github.com/solana-labs/solana-program-library/blob/acabef3680e972baf8e9a47531c35eaa45405740/token/program-2022/src/serialization.rs#L112-L147, and I imagine we'll want to stick with that

[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