beacon/engine, eth/catalyst: encode payload body access lists

[0xoasis]

Summary

• encode ExecutionPayloadBodyV2 block access lists as RLP bytes before JSON serialization
• preserve null for pre-Amsterdam blocks and pruned access lists
• update the regression test to require 0xc0 for an empty RLP list

Motivation

The Amsterdam Engine API defines ExecutionPayloadBodyV2.blockAccessList as DATA|null containing an RLP-encoded block access list. The V2 implementation exposed bal.BlockAccessList directly, producing structured JSON such as [] instead of hex DATA such as "0xc0". This gives Engine API callers an incompatible response shape.

Specification: https://github.com/ethereum/execution-apis/blob/main/src/engine/amsterdam.md#executionpayloadbodyv2

Validation

• go test ./eth/catalyst -run TestGetPayloadBodyV2BlockAccessList -count=1
• go test ./beacon/engine ./eth/catalyst
• make all
• go run ./build/ci.go test -short
• go run ./build/ci.go test
• go run ./build/ci.go lint
• go run ./build/ci.go check_generate
• go run ./build/ci.go check_baddeps

[0xoasis]

Closing as superseded.

The RLP encoding of ExecutionPayloadBodyV2.blockAccessList as hex DATA (and the "0xc0" regression expectation) has already landed on master via related engine API work (see #35372 and surrounding payload-body changes). After merging master, this branch has no remaining unique diff.