[crypto] Decide on the right serializations of multisig

[benr-ml]

1. Some externally used types like CompressedSignature and PublicKey depend on the Rust enum's serialization while others like SignatureScheme depend on specific byte represented flags.
2. pub enum PublicKey exposes fastcrypto's internal types.

We should decide if the above is ok and make sure that the TS libraries & documentation are consistent with that.