Expose ENR custody columns

[dapplion]

Issue Addressed

Part of

• DAS - Tracking Issue #4983
• Progressively merge das to unstable #6072

Proposed Changes

• Add custody_columns method to NetworkGlobals

[pawanjay176]

Just a small suggestion wrt Enr::custody_subnet_count, I think we should make that simply return an error on invalid/non existing enr instead of returning the default value. Basically change the function signature to

fn custody_subnet_count<E: EthSpec>(&self, spec: &ChainSpec) -> Result<u64, &'static str>;

Swallowing the error at the accessor function and returning the default might lead to confusing errors.

[jimmygchen]

Good that you raised this again - we actually thought about this when this was initially implemented, and the original reason to swallow it is wanting to have the fallback logic in one place, and don't want to do any error handling due to invalid input from peers, but maybe there's better approach and may even be useful to log an error?

[pawanjay176]

We could just do the fallback logic at the calling site. The accessor function on that trait doesn't seem like the right place to do fallback logic.
Consider a situation where a client has a bug in their encoding for the enr custody subnet field, in that case, we would always swallow the error which isn't what we want to do.

[jimmygchen]

The main das branch has been merged and this change has already been included.