Impl TreeHash for alloy::rpc::types::beacon::BlsPublicKey a.k.a alloy_primitives::FixedBytes<48>

[David-Petrov]

At my team, we have a use case where it would make sense to hash a BLS public key as a part of a proxy delegation message. Seeing as it's a type alias for FixedBytes<48>, an implementation for that primitive type would suffice (modulo versioning issues, so I'm not sure what the best approach would be).

I think this would be a convenient addition to the trait's implementations in general.