Add subxt like generation of the Call enum

[athei]

The call_runtime API offered by pallet-contracts allows contracts to pass an encoded Call to the runtime which then executes it.

The API still has a few issues:

• It is unstable (it will stay unstable if we don't start tinkering with it)
• Dispatchables can't return data on Ok. This is a constraint in FRAME. But I think it can be changed (I am looking into this).
• We don't have support for call_runtime in ink!, yet ( Support call_runtime #1641).

As of right now those call data structures need to be constructed manually which is quite cumbersome. We need some code generation that constructs the Call enum of a runtime from metadata which can then be passed to call_runtime.

The code for this should already exist within subxt. We need to make it conveniently accessible within ink!.

This is the inverse direction of #1674.

[pmikolajczyk41]

I've already started looking at it some time ago and I'm planning to work on that next week as a follow-up to the mentioned PR :)

[athei]

call_runtime will be deprecated in favour of execute_xcm as the Call is not stable enough to use it in contracts. Hence we don't want to build more codegen around it. We rather double down on XCM.