merge go-eth2-client upstream

[pk910]

No description provided.

[redpandabot]

PR #55 merges the go-eth2-client upstream tail: it adds compile-time interface-assertion files to http/mock/multi/testclients and updates mock's SubmitProposal (to take *api.SubmitProposalOpts) and renames SubmitBLSToExecutionChange to the plural SubmitBLSToExecutionChanges (taking a slice) to match the merged interfaces. I verified every asserted interface name exists in service.go and, via a per-package signature match (no Go toolchain available), that each asserted interface is actually satisfied by the asserted receiver types; all hold. I also confirmed no stale callers/signatures of the old methods remain. The change is self-consistent and additive; nothing to flag.

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_{Reviewed 6 changed file(s) @ 72bf95ed — no blocking issues found.
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