Support using native tokens for proposal deposits

[JakeHartnell]

Some DAOs may want to use native tokens like Juno for proposal deposits.

This would work well for more open "anyone can submit" style DAOs like in #386 to help fight spam or as part of the process for raising a treasury.

[0xekez]

Have done this work for cw-plus here: CosmWasm/cw-plus#751

Feel like it came together reasonably nicely. Should probably reuse this design.

[mikedotexe]

Thanks for the link @ezekiiel! I tried reading that PR but will admit it didn't click immediately. Nonetheless, I believe we can take a stab at this. Perhaps I'll follow up with questions in Discord if we get stuck. 🙏🏼

[0xekez]

Great! Thanks @mikedotexe! Discord is a great place to ask questions. Happy to help.

[0xekez]

#481