feat: implement fulu enr support

[tosynthegeek]

What was wrong?

Implements the remaining Fulu discovery ENR changes required by spec.

Previously, discovery ENRs always advertised the Electra fork digest and did not account for upcoming fork.

How was it fixed?

• Made ENRForkID generation fork-aware instead of hardcoding Electra.
• Updated ENR construction to compute the current fork digest from the active fork schedule.
• Added support for the Fulu discovery ENR fields required by the specification.
• Computed and advertised the next fork digest (nfd) when applicable.
• Updated discovery ENR generation to reflect current and upcoming forks.

To-Do

• I have read CONTRIBUTING.md.
• This PR title follows conventional commits.

[shariqnaiyer]

Looks good