Implement pallet that extends our current treasury pallet with buyout feature

[bogdanS98]

Closes #395.

Overview

This pallet allows users which hold other assets than our native asset to exchange them against the treasury in return for our native. This is limited by the assets supported, minimum and maximum buyout amounts and buyout period (after which the total buyout amount for a user is reset).

Buyout limit can only be changed by root. If set to None, then there is no amount limit for buyouts.

This pallet was only added for Foucoco runtime in this PR.

Assets supported

Amplitude

• USDT
• KSM

Pendulum

• USDC
• USDT
• DOT
• GLMR

Notes:

1. We still have to properly define the values of the following parameters for each runtime:

• SellFee
• MinAmountToBuyout
• BuyoutPeriod

2. This pallet has yet to be tested in Polkadot.js

[ebma]

I left some comments.

Overall I think we should never use '+,-,*,/' operators directly and rather use defensive operations like 'saturating_..' or 'checked_..'.
And I think we can remove the tight coupling to the oracle pallet.

Besides that we also need the logic for the signed extension but I know you are aware of this and it's still a WIP PR after all.

[ebma]

Nice changes 👍 I think it would have been fine to just make the extrinsic such that the list is totally replaced, ie. implementing a setter and ignoring the last state. But it's fine either way 🤷

[gianfra-t]

I checked the changes since last review and everything looks ready! I just left a comment but I don't think it is a blocker.

[ebma]

I think it's good (better now with the simpler extrinsic). Just left a comment on the benchmark which is more an FYI, I don't mind if you don't change it @bogdanS98 and a question.

[ebma]

I think it's ready for merge now 👍