Asset pallet move files

[HenriqueNogara]

This PR doesn't change any existing logic, it only moves things around.

• The asset pallet error was moved to a dedicated file;
• All type definitions were moved to a dedicated file;
• Functions that were not being used were removed;
• Organize asset lib file in blocks: All base functions, All validattion functions, All Storage Writes, All RPC functions, and Trait implementation;
• Add more unit tests to register_ticker and accept_ticker transfer;

changelog