Extension to Treasury Pallet for Assets

[joepetrowski]

This would be one step toward #98 and also related to https://forum.polkadot.network/t/a-better-treasury-system/291.

We should have an extension to the Treasury pallet, similar to how pallet-asset-tx-payment extends pallet-transaction-payment. It should:

• Allow spend proposals with AssetId.
• Use the same AccountId as the Treasury's Pot.
• Provide a Trait to convert from BalanceOf to AssetId and back. The default implementation can use the ratio of minimum balances.
• Take deposits based on the conversion.
• Interpret the SpendOrigin based on the conversion.

Payout should assume that the pallet's AccountId already has enough of the asset, and would fail if it does not. That is, this extension should not worry about acquiring AssetId.

[jsidorenko]

What if we create a separate struct like AssetOrNative that will keep inside the information about the assetId or currency and the amount? I would be glad to use it inside the uniques-pallet so that users could set the price for their NFT in a different token. But in my case, there won't be a conversion needed, which means if the price is set in USDC, then it will be the only token to pay for the NFT.

[joepetrowski]

Closed by #1333