Implement new deposit module interface.

[0xekez]

This implements the proposal deposit interface described in https://github.com/DA0-DA0/dao-contracts/discussions/462. It is based on @JakeHartnell's work on proposal deposits in https://github.com/DA0-DA0/dao-contracts/pull/465.

At the moment, this only implements the interface on cw-proposal-single. I'll add additional commits which implement a deposit module and test it as this comes along. Putting up so people can follow along if they'd like.