Add Fulu boilerplate

[macladson]

Proposed Changes

Adds Fulu fork boilerplate.

Additional Info

This makes no attempt to unify Fulu with PeerDAS. This will need to be a separate PR.

[dapplion]

@macladson I've applied the fork_enabled pattern on the remaining places where it is possible in this commit a543765 please review and consider cherry-picking into this PR

To motivate my case against the match fork_name:

• Relying on the type system to direct our dev attention to certain places would make sense ONLY if we add a new variant to ForkName once the fork is fully spec'ed. Otherwise (like here, now) we just fill all those match cases assuming the fork is empty, losing the all benefits of this approach. Let's accept that we are unlikely to wait for a full spec before adding the boilerplate.

[macladson]

Thanks for the commit! I agree with your reasoning and I've cherry-picked it into the main branch.

I ended up reverting the web3signer.rs changes as it actually uses a local variant of ForkName not types::ForkName so to remove that boilerplate, we'd either need to write a mapping from types::ForkName => web3signer::ForkName or rewrite the web3signer to use types::ForkName. I can't comment on why types::ForkName was chosen in the first place, but either way, it feels out of scope for this PR.

[dapplion]

Looks great! Send it