refactor: move serialisation out of ValidatorSigner

[pugachAG]

Currently we add a method to ValidatorSigner for every data type we want to sign. I'm not sure what exactly is a benefit of doing that, but such approach results in quite a few annoying issues:

• Overly verbose code: need to add 3 methods for every data type to be signed.
• Not consistent with signature verification: we just call verify method on public key passing signature and bytes, why should signing be any different?
• It blows up ValidatorSigner public API making it awkward to work with.

This PR repurposes sign_chunk_contract_accesses to be a generic sign_bytes method and replaces usage for witness contract code distribution related types. The rest will be addressed in a separate PR.