fix(baseapp): vote extensions on first block

[facundomedica]

Description

Issues appear when starting a new chain with vote extensions enabled from the first block.

Access to first block uncommitted state during ExtendVote and VerifyVoteExtension

ERR CONSENSUS FAILURE!!! err="consensus key is nil: collections: not found: key 'no_key' of type...

This was happening because we'd always try to read from disk, but during the first block nothing has been committed yet so no data is accessible. So we have to allow access to the uncommitted state (finalizeBlockState), which contains changes done during InitChain (only on the first block, just like PrepareProposal and ProcessProposal).

Error during validation of vote extensions when height == VoteExtensionEnableHeight

vote extensions enabled; received empty vote extension signature at height 2

During the first block in which vote extensions are enabled, RequestPrepareProposal won't contain any vote extensions (because they should be from H-1). So here we must only check for vote extensions when
height > VoteExtensionEnableHeight

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[github-actions]

@facundomedica your pull request is missing a changelog!

[alexanderbez]

LGTM!

[likhita-809]

nice!