Fulu support

[olegshmuelov]

No description provided.

[Copilot]

Pull Request Overview

This PR adds support for the "Fulu" Ethereum upgrade by integrating the new fork into the beacon block signing functionality and adding corresponding test data for various block types.

• Added case handling for spec.DataVersionFulu in both blinded and regular beacon block signing functions
• Updated the go-eth2-client dependency to a pre-release version that includes Fulu support
• Added comprehensive test data files for all supported block types in the Fulu format

Reviewed Changes

Copilot reviewed 12 out of 16 changed files in this pull request and generated no comments.

File	Description
signer/sign_blinded_beacon_block.go	Added Fulu case to handle new fork version in blinded beacon block signing
signer/sign_beacon_block.go	Added Fulu case to handle new fork version in regular beacon block signing
go.mod	Updated dependencies to support Fulu upgrade with pre-release go-eth2-client version
signer/testdata/blocks/*.json	Added test data files for various block types (phase0, altair, bellatrix, capella, deneb, electra) to support testing

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