[confidential-transfers] Use direct constructors for the ElGamal pod types with agave 2.1

[samkim-crypto]

The zk-sdk that ships with agave 2.0 does not provide an easy way to directly construct pod types like PodElGamalPubkey or PodElGamalCiphertext from bytes. As a workaround, in token-2022 and in the confidential transfer ciphertext arithmetic crates, the bytes are converted to strings and then the strings are converted to these pod types (using the fact that the pod types implement FromStr).

Proper constructors for the pod types in the agave repo has been merged (anza-xyz/agave#2711). Once agave 2.1 is released, remove the FromStr workaround and directly use the provided constructors.