Allow readers to ignore trailing bytes after declared SSZ bytes

[dapplion]

Description

Trailing bytes after the declared n SSZ bytes cannot change the decoded message, but rejecting them requires observing EOF before processing, which adds latency and lets a peer stall a request by holding the stream open. This makes ignoring them permitted.

Teku rejects trailing bytes; Lighthouse, Prysm, Lodestar, Nimbus and Grandine accept them. MAY leaves all six conformant.

Related to sigp/lighthouse#9301
Related to sigp/lighthouse#9567

[Alleysira]

Thanks for looking into this @dapplion. I would like to add some cross-client context for discussion, since the implementation has changed after the original report:

• Teku already reject it, so no report
• lighthouse in favor of a spec change here (Trailing Bytes Spec Violation sigp/lighthouse#9301)
• nimbus has since changed to reject with (RPC chunks with stricter DOS limits where possible status-im/nimbus-eth2#8744)
• grandine's report here(Trailing Bytes Spec Violation grandinetech/grandine#686)
  • with a pending fix (Reject RPC requests with trailing bytes after the SSZ payload grandinetech/eth2_libp2p#28)
• prysm no resposne yet (Trailing Bytes Spec Violation OffchainLabs/prysm#16782)
• lodestar no resposne yet(Trailing Bytes Spec Violation ChainSafe/lodestar#9362)

[jtraglia]

Will merge after at least one developer from each client team has approved this.