Add hooks to x/gov OnVote

[ValarDragon]

Summary

Currently there are no hooks on x/gov. However, it would be useful to have hooks for when a user votes on a proposal, as an application may want to provide additional incentivizes for voting users on certain proposals.

(I think we should separately add more hooks for once a proposal has finished deposit period, and a tally event occurs)

Proposal

Add the following hook that executes after a user has voted.

AfterUserVoted(Context, AccAddress, VoteType)

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For Admin Use

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[fedekunze]

proposal ACK. Can you name some use cases for this improvement? How would you imagine this being used by different chains? @sunnya97

[sunnya97]

Osmosis is using this specific hook in order to reward users for casting a vote, and so our module giving this reward needs to receive a hook whenever a vote is cast

I can imagine other use cases as well, such as one that keeps track of how many votes have been cast so far, and to perhaps end a voting period early if certain conditions are met.