Asset manager release checklist

[0x8f701]

• [x] Make name consistent, we are using AssetRegistrar as trait name, will be good to use asset-registrar for pallet name to make it consistent
• [x] mv hard coded pallet id to runtime as a pallet config
• [x] mv AssetRegistrar trait to primitives/src/xcm.rs
• [x] rename primitives/src/xcm_gadget.rs to primitives/src/xcm.rs
• [ ] simplify migration code to insert only units per second & asset id <-> multilocation mapping
• [x] register_asset make metadata mandatory
• [ ] version management of multilocation passed in parameters

[yrong]

About the migration the new pallet should work side by side with the legacy foreign assets, so I would suggest we keep this pr focused and move that to future pr.

[yrong]

Remove AssetRegistrar trait since we manually created new asset so do not need

[yrong]

Since without the VersionedMultilocation we can still fulfill the future migration, and the register call is usually not used directly by frontend, so we keep it simple here just use Multilocation