cometbft: Add BLS12-381 support

Follow-up to https://github.com/cometbft/cometbft-rs/pull/82

https://github.com/cometbft/cometbft-rs/pull/82 brought a v1.0.0 support, which includes BLS12-381 curve (under a feature flag). We should add a support on the Rust side as well (similarly under `bls12381` compile flag).