Gloas: payload envelope in `PreProcessingSnapshot`

[michaelsproul]

Description

Closely related to:

• Gloas: track head envelope in canonical_head #8956

As noted by Mark here, it probably makes sense to have the parent payload envelope available during block processing:

• https://github.com/sigp/lighthouse/pull/8886/files#r2880060015

Just need to consider the full implications of this, but it should be fairly straight-forward.