feat: asset management

[Daanvdplas]

Implements the functionality for asset management:

• create
• start_destroy
• set_metadata
• clear_metadata
• contract example with creating an asset in the constructor or linking the contract to an existing asset.

The dispatchables required to completely destroy an asset:

• destroy_accounts
• destroy_approvals
• finish_destroy
  , have not been included in the api because they do not have to be signed by the owner of the asset. In addition, it requires configurations of RemoveItemsLimit (originally from pallet assets) for the api::fungibles pallet to specify the max. # of accounts and approvals that can be removed in a single tx by a contract. This is all not needed if asset owners (in this case via a contract) are required to complete the steps of destroying an asset directly via the pallet (this is normally also the case). Only starting the destroying process of an asset has to be initiated by the contract if it is the owner of the asset.

Closes #92