Migrate xpallets-assets to pallet v2 (#560)

* feat: init xassets migration

* chore: process xassets migration, compliation error need to be resolved

* Migrate xpallet-assets to attribute macro

* fix(mocks): assimilate_storage

* fix: weights

* fix: restore the initial settings of weight

* fix: optimize import GenesisBuild and add apllet-event comment

* style: amend some comments