Allow Asset Conversion Subscriptions

[joepetrowski]

In support of a multi-asset treasury, the Asset Rate pallet needs updates about the ratio of its budgeting unit (on Polkadot, DOT) and other assets that are acceptable for proposals.

This pallet contains several origins, most notably the Update Origin, which can update the ratio for a given asset. The location with the Asset Rate pallet should be able to subscribe to regular updates over XCM from a location with a means of providing updated rates, in this case the Asset Hub with the Asset Conversion pallet.

This could be a general "Subscriptions pallet", but for the immediate use case, the most important is to be able to schedule regular messages (say, every hour) for a set of assets.

[bkchr]

Sounds like it could use #606?

[joepetrowski]

Yeah I had not seen that. In this case though it doesn't need a (k, v), it needs the return of the get_reserves runtime API so that it can convert the (Balance, Balance) return to a rate.