[Multi-Asset Treasury] Implement treasury pallet to be more generic

[tonyalaribe]

Motivation

The treasury pallet can be made generic over asset classes, and support spends on arbitrary asset classes which might be in the possession of the treasury.

Request

To do this, the treasury pallet can be implemented over the Pay trait (paritytech/substrate#13603) and over the Exchange Oracle trait (#202) to allow us have a pallet that can be implemented to support spends on any supported asset class, and not just the natively supported asset class.
Doing this means the treasury pallet does not need to be tied to the fungibles or fungible traits and can be more flexible in how payments are made with the treasury pot, or even where said funds are held.

This is a step towards #98
More info about the larger plan is on this post: #98