Add payload_status field to AttestationData

[mehdi-aouadi]

This PR proposes a structural clarification to the AttestationData container by renaming the index field to payload_status. This field has been unused for its original purpose (committee index designation) since the Electra upgrade and is now repurposed to explicitly signal the status of the execution payload of the attested-to block. The underlying uint64 type is retained to ensure structural stability and provide extensibility for future payload statuses.

The continued existence of the attestation_data.index field in its legacy form presents several issues:

• Speci clarity: The name index strongly implies a committee index, a function it no longer serves (hardcoded to 0). This discrepancy is a source of confusion for client implementers, researchers, and future protocol developers.
• Inconsistent usage: Using a field named index to convey information about payload status is a form of semantic overloading that is architecturally unsound and prone to implementation errors. A dedicated, clearly named field is preferable.
• Technical debt: The index field was intentionally left in place during the Electra hard fork to de-risk the introduction of other complex changes when implementing EIP-7549. Now is a good opportunity to address this known technical debt before further protocol changes build upon this ambiguity.

Risk: Any modification to AttestationData can have wide-ranging and subtle effects.
Mitigation: This change is strictly a rename and does not alter the size, SSZ structure, or serialisation of the container. By avoiding a type change (e.g., to an enum), we minimise the scope of downstream code changes and reduce the risk of introducing new bugs.

Related Links
Context: EIP-7732 Breakout Call #25
Discussion: Discord Thread on Ethereum R&D

[jtraglia]

Since PayloadStatus is not an SSZ type, I think it would be better to rename the field and use a boolean.

Maybe payload_present or payload_revealed (not sure which is better):

Suggested change

	payload_status: PayloadStatus
	payload_revealed: boolean

[mehdi-aouadi]

In the end I just renamed the index field to payload_status and kept the uint64 type to avoid impacting the SSZ serialisation. Plus it would be possible to introduce new statuses if needed in the future...

[mehdi-aouadi]

Closing this PR since it was discussed during EIP-7732 Breakout Room Call #26 and the majority preferred implementation stability and judged this change complex to implement