Add support for multiple rust-kzg backends

[sauliusgrigaitis]

rust-kzg supports multiple ECC backends, however, the current approach is by manually switching backends in Cargo.toml file. Obviously, it's not convenient. We should have an option to specify at compile time which ECC backends we want to compile in and then allow select one at runtime (via command line flag). Similarly, like #61 for BLS signatures.

It's likely that the best place to do it is https://github.com/grandinetech/rust-kzg/tree/main/kzg . In such case kzg crate needs to refactored in order to get rid of blst dependency. If we must use some particular ECC backend type (blst currently) then we need to use a highly portable ECC backend (pure Rust). Otherwise, if kzg crate is not a good place for multibackend support then we should do it in a wrapper in Grandine.

@ArtiomTr