[cryptography] Add Multi-Signature `Scheme`

[patrick-ogrady]

To provide first-class support for multi-signatures, we should consider implementing a dedicated Scheme (like we did for batch verification with BatchScheme):

monorepo/cryptography/src/lib.rs

Lines 93 to 129 in cee6569

	/// Interface that commonware crates rely on for batched cryptographic operations.
	pub trait BatchScheme {
	/// Create a new batch scheme.
	fn new() -> Self;
	/// Append item to the batch.
	///
	/// The message should not be hashed prior to calling this function. If a particular scheme
	/// requires a payload to be hashed before it is signed, it will be done internally.
	///
	/// A namespace should be used to prevent replay attacks. It will be prepended to the message so
	/// that a signature meant for one context cannot be used unexpectedly in another (i.e. signing
	/// a message on the network layer can't accidentally spend funds on the execution layer). See
	/// [union_unique](commonware_utils::union_unique) for details.
	fn add(
	&mut self,
	namespace: Option<&[u8]>,
	message: &[u8],
	public_key: &PublicKey,
	signature: &Signature,
	) -> bool;
	/// Verify all items added to the batch.
	///
	/// Returns `true` if all items are valid, `false` otherwise.
	///
	/// # Why Randomness?
	///
	/// When performing batch verification, it is often important to add some randomness
	/// to prevent an attacker from constructing a malicious batch of signatures that pass
	/// batch verification but are invalid individually. Abstractly, think of this as
	/// there existing two valid signatures (`c_1` and `c_2`) and an attacker proposing
	/// (`c_1 + d` and `c_2 - d`).
	///
	/// You can read more about this [here](https://ethresear.ch/t/security-of-bls-batch-verification/10748#the-importance-of-randomness-4).
	fn verify<R: RngCore + CryptoRng>(self, rng: &mut R) -> bool;
	}

The delta from BatchScheme would be pretty small (during add we wouldn't provide signature and instead would provide an aggregate signature during verify). We may also want to have a combine function that aggregates signatures.