refactor(eth2api): making only 1 DataVersion and BuilderVersion

[iamquang95]

Remove the DataVersion and BuilderVersion in eth2util.

[emlautarom1]

Could you elaborate on what has changed? How is it that we can merge these two types?

[iamquang95]

@emlautarom1, previously in the signeddata PR, I ported both DataVersion / BuilderVersion definitions from Charon: one from go-eth2-api and one from eth2util. The eth2util copy existed because signeddata still uses the legacy numeric serialization format for these fields.

This PR removes that duplication. We now keep a single canonical version type in eth2api, and preserve the legacy signeddata encoding through serde helpers rather than through a separate enum type.

The main benefit is that it reduces the amount of version/conversion logic we need to maintain. We no longer need to keep two nearly identical types in sync, and any new serialization behavior can now be introduced in one place.

[emlautarom1]

Thanks for the explanation. LGTM.