feat: add MsgWithdrawProposal to group module

[atheeshp]

Description

Closes: #9660

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[aleem1314]

lgtm

[blushi]

Just some comments re: simulations

[atheeshp]

@blushi can you have another look?

[blushi]

Looks good to me! But I'd wait for the simulations to get fixed on master before we merge this.