Use single library for SSZ and fix bls-to-execution-change cmd

[mksh]

This is pre-requisite for https://github.com/ChorusOne/eth-staking-smith/issues/53

The command bls-to-execution-change was not working before, so this adds signature tests and verifies action works with real Holesky validator https://holesky.beaconcha.in/validator/a7c6468e36b0cc23580fd5c4e44b5505db677cfeb2692a5d7992bddac4706981f0bdaf568d565703d6e68e0211912349#deposits

A future pre-signed exit message command will use the same genesis validator root values, and reuse most of signing logic, so it is important to cleanup bls-to-execution-change before adding new functionality.

[mksh]

The most important part of cleanup is removing ssz-rs fork https://github.com/ChorusOne/ssz-rs and switching to use Lighthouse SSZ libraries everywhere.