Support pallet-assets sufficients as account providers

[acatangiu]

Example scenario: on AssetHub we should be able to allow accounts to be created and to hold non-sufficient assets even without holding DOT ED, as long as they hold other sufficient assets (e.g. USDT or USDC).

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

This PR is meant to explore options to make that possible.

The first commit showcases a very simple UT failing because of above.
The second commit comments on the relevant functions that currently allow an account to exist with only e.g. USDT, but do not allow same account to hold any other non-sufficient asset.

[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).

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs.
Job name: test-linux-stable 3/3
Logs: https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/6945441

[acatangiu]

moving this to #5298