chore: Update ValidateVoteExtensions

[alexanderbez]

Description

Update ValidateVoteExtensions to ensure at least 2/3 validators, by voting power, have submitted valid vote extensions as specified by the SDK ABCI++ ADR.

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[tac0turtle]

something that came to me while reading this was what if an application says only these 5 validators need to do this vote extension and these other 10 have to do some other work, would that make sense to try or does that defeat the purpose a little?

[alexanderbez]

If they'd wanna do something like that, then I would argue they should implement their own ValidateVoteExtensions function. Recall, ValidateVoteExtensions is just a helper function we provide for apps. They're not forced to use it. They can also use it as a reference too if they wish.