Runtime: Budget reduce proposal - triggerable n times per term for up to some % of treasury holdings

Note: this is just a GH issue describing a possible way to adjust the current "reduce council treasury" proposal. It has not been approved by the council and may not be of interest.

# Problem
Outlined here: https://pioneerapp.xyz/#/forum/thread/903

# Solution
One of the issues describing how to implement this functionality (https://github.com/Joystream/joystream/issues/4684) mentioned:
> One thing that may be interesting to have is a proposal that can be triggered `n` times per council period and can burn some max % of the council mint.

In this case I think this solution may be a way to improve the current proposal type as it would limit the amount of tokens that can be burned and places the onus on adjusting inflation upon the "budget increment" proposal type which requires more consensus than this proposal type,