Use fungible token traits in alliance pallet

[KiChjang]

Replaces the ReservableCurrency trait bound on the Currency associated type for pallet_alliance::Config with fungible::BalancedHold.

Related to #8285 and paritytech/polkadot-sdk#327.

I may have jumped the gun here as the logic for BalancedHold::slash_held is different from ReservableCurrency::slash_reserved, so we may instead have to fix that first.

[joepetrowski]

In general I think moving to these traits is a good thing. However, there are some runtime features that interact with reserved/slashed assets, e.g. https://github.com/paritytech/cumulus/pull/1433/files, that we're quite far into testing locally and on Westend. I'd prefer to launch the Collectives chain using the balances as they are now because we have high confidence that they are safe. Then we can give this more attention, auditing, and testing and migrate to it in an upgrade.

[stale]

Hey, is anyone still working on this? Due to the inactivity this issue has been automatically marked as stale. It will be closed if no further activity occurs. Thank you for your contributions.