[pallet-referenda] Crowd Sourced Decision Deposit

[ggwpez]

Currently the decision deposit for a referendum must be paid in full by a single address. We should add a mechanism that allows it to be crowdsourced. @bkchr suggested that we can re-use the code of the crowdloan-pallet from the Polkadot runtime. I am not familiar with the code of tha pallet but probably needs a new pallet though to manage the deposits.

Requirement:

• It should be possible to split the payment between different addresses (at least 128)
• The code should only consider the n highest amounts paid and refund the lower ones

[runcomet]

Hi @ggwpez, I would like to work on this! Decision deposits could help mitigate some whale activity in OpenGov.

Assigning me would be nice if possible

[ggwpez]

Thanks @runcomet!