pallet-assets sufficient assets ED should provide same benefits as DOT ED

[acatangiu]

Holding a sufficient pallet-assets asset satisfying ED, should allow that account to also receive other assets (in technical terms, it should be an account provider).

Example scenario: on AssetHub if an account holds USDT/USDC (enough to cover ED), it should be able to receive/hold other assets too, without necessarily having to also hold DOT ED (as it requires now).

This is not possible now though because pallet-assets "sufficient" assets do not act as account providers.

Here is a branch with minimal repro of the issue (very simple unit-test), diff against master here.

[acatangiu]

I looked into changing pallet-assets to add inc_providers() alongside inc_sufficients(), but then the current logic for killing accounts (defined as (asset, account) pair) doesn't really support failing to kill sufficient when consumers > 0 (aka not allow USDT withdraw if it is sole account "provider" for other existing (asset, account) pairs).