[pallet-assets] add asset_exists(id: AssetId) function (#12782)

* check if an asset exists via loose coupling

* Update frame/support/src/traits/tokens/fungibles.rs

Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>

Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>
Co-authored-by: parity-processbot <>