feat: NextAssetId

[Daanvdplas]

In the 1.14 (#198) upgrade pallet assets introduced a NextAssetId.

When it is configured to a value it results in having the pallet incrementally update the NextAssetId when an asset is created. The flow would be; check the NextAssetId and use the queried id as parameter when creating an asset. In stead of choosing a random value and see if it is already taken.

Right now it is set to None which will results in how it previously worked.

This might become useful for #147