feat: Add bls12381 signing provider

[garrysg]

• add generic bls12381 signing (min-sig + min-pk)
• add tests using test vectors from ethereum's repo

[rnbguy]

Thanks a lot for the PR 🙌🏼 Left few comments. Please add a test that generates a private key, signs a message, and verifies the signature with the public key.

Would appreciate if you filled in bls12381_min_pk.rs too.

Would be great if you could merge these two structs Bls12381<T> and have two implementations at Bls12381<MinPk> and Bls12381<MinSig>.

[garrysg]

@rnbguy has refactored BLS12-381 signature scheme support with minimal-signature and minimal-public-key variants using a type-parameterized generic design

[rnbguy]

can we have something like this ?

// bls12381.rs
pub trait BlsVariant: Copy + 'static {
    type SecretKey; type PublicKey: Clone; type Signature: Clone;
    const PK_LEN: usize;
    const SIG_LEN: usize;
    const DST: &'static [u8];
    fn verify(sig: &Self::Signature, msg: &[u8], pk: &Self::PublicKey) -> BLST_ERROR;
    // + from_bytes/to_bytes for each type
}

// impl `SigningScheme` for any `Bls12381<V: BlsVariant>`
impl<V: BlsVariant> SigningScheme for Bls12381<V> {
    type Signature = Signature<V>;
    type PublicKey = PublicKey<V>;
    // ...
}

// impl `SigningProvier` for `BlsProvider<V: BlsVariant>`
impl<C, V: BlsVariant> SigningProvider<C> for BlsProvider<V> ... { ... }

impl BlsVariant for MinPk { /* PK=48, SIG=96, blst::min_pk::* */ }
impl BlsVariant for MinSig { /* PK=96, SIG=48, blst::min_sig::* */ }

then we can directly use,

// minpk
type Bls12381MinPkScheme = Bls12381<MinPk>;
type Bls12381MinPkProvier = BlsProvider<MinPk>;

[garrysg]

@rnbguy Refactor BLS signing to a generic BlsVariant/Bls12381 with shared tests, eliminating duplication while keeping MinSig/MinPk type-safe and ergonomic.

[rnbguy]

Thanks for the PR 🙏🏼