Split off asset registration from add_asset

[mattsse]

Issue summary

• The add_asset call was initially designed to also execute asset registration. This has the drawback that, the MultiLocation of the asset must always be included in the proposal that will execute the call even if the asset is already registered.
• This makes the register_asset call redundant Redundant register_asset extrinsic that can be used to register asset with PINT_ASSET_ID #369
• To make this more convenient and less error-prone (MultiLocation), the asset registration part should be made separate so that:
  • add_asset requires that register_asset was called before once
  • add_asset does not require the MultiLocation anymore

Other information and links

• Redundant register_asset extrinsic that can be used to register asset with PINT_ASSET_ID #369