Democracy: allow a single account to cast multiple different votes

[xlc]

We are working a staking liquidity protocol which our parachain will be staking/nominating on relaychain on behalf of parachian users in a fully decentralized manner.

We don't want to take the users ability to participate governance when they stake via our parachain. So I am designing a system to allow the parachain to participate democracy on behalf of parachain users.

In our design, our parachain account will hold user's DOT on relaychain, and stake & nominate them. This account can also perform voting on relaychain. Our parachain account represents all parachain users which observably will vote differently (aye/nay, different conviction). But the pallet-democracy only support one account to cast one kind of vote. We can't use sub account because the DOTs will be bonded and non-transferable so it won't be to update the balances on each sub account.

So we need ability for a single account to cast different votes with different conviction and bonding amount.

I believe most of the custodial service will have similar requirement at some stage.