call asset_exists function in pallet-asset-registry

[bernardoaraujor]

Motivation

pallet-asset-registry is currently using T::Assets::minimum_balance(asset_id).is_zero() as a way to verify whether some Asset exists on Trappist's pallet-assets.

That is a bit hacky, and served as motivation for paritytech/substrate#12444

Suggested Solution

Ever since paritytech/substrate#12782 was merged, we can replace

trappist/pallets/asset-registry/src/lib.rs

Line 150 in 5636953

!T::Assets::minimum_balance(asset_id).is_zero()

with a simpler call to asset_exists.

[bernardoaraujor]

paritytech/substrate#12782 only got into the release pipeline after polkadot-v0.9.35

so that's also a prerequisite: Trappist needs to be using substrate on polkadot-v0.9.35 or later

[evilrobot-01]

So glad that was finally added.

[stiiifff]

Depends on #122

[stiiifff]

Closed by #131