ePBS/Gloas: SSV patches (failover + prysm workaround) on the gloas base

[iurii-ssv]

Part 1 of ssvlabs/ssv#3014 — bring Gloas (ePBS / Glamsterdam) into our go-eth2-client dependency.

What

gloas-support = the upstream attestantio/go-eth2-client gloas branch (the base of this PR) + our two long-standing SSV patches re-applied on top:

• multi: prefer higher-scored clients on failover — the reputation-score failover behaviour (soft-penalize a client instead of hard-deactivating it; also fail over on a context deadline).
• http: tolerate an empty Eth-Consensus-Block-Value header — the prysm workaround. Still required: the gloas branch's populateProposalDataFromHeaders does not carry the empty-value guard.

The base branch gloas mirrors the upstream gloas tip, so this PR's diff is only the two patches (3 files).

Why this shape

main is v0.27.0-based (+ upstream Fulu + these two patches); the gloas branch is v0.29.0-based. They share only the v0.27.0 merge-base and carry divergent Fulu histories, so we branch off gloas and re-apply the two patches rather than merging gloas into main. The three touched files are also reformatted upstream (v0.27→v0.29), so the re-apply resolved small conflicts; the semantics match the originals, expressed in the gloas code style.

Validation

• Build / vet / gofmt / nlreturn+whitespace / multi unit tests: green.
• Generated ssz_static vectors from consensus-specs v1.7.0-alpha.13 (the spec devnet-8 pins) and ran them against these types: all 58 spec types pass SSZ round-trip + HashTreeRoot — progressive merkleization is correct for devnet-8, with no devnet-7→devnet-8 SSZ skew.
• The branch's self-contained sszroot_test.go (independent v1.7.0-alpha.12 PySpec roots) passes.
• Note: upstream's consensusspec_test.go reports YAML-only mismatches against these vectors (uint64s rendered as decimal strings vs the vectors' hex; a subcommittee_index yaml tag; an empty-transaction edge case). These are serialization-harness artifacts and do not affect SSZ bytes, signing roots, or the beacon-API JSON path.

Consuming from SSV

Point the replace at the head branch:

go get github.com/ssvlabs/go-eth2-client@gloas-support && go mod tidy

This lands DataVersionGloas and the correct spec/gloas types.

Preview pin — track upstream

The upstream gloas branch is unmerged and still moving (open PR stack: attestantio#302, attestantio#312, attestantio#313, attestantio#316, attestantio#318). Treat this as a pinned preview; refresh the gloas base and re-apply/rebase these two patches as upstream advances, and re-pin to a tag once gloas merges to master and ships in a release.

[iurii-ssv]

Superseded by #19. Renamed to a consistent scheme: the mirror is now upstream-gloas and the patched snapshot is fork-gloas (same content). Closing and removing the old gloas/gloas-support branches.