Closes #79

Introduced changes

• Changed token_uri() mutability to view
• Created ERC721MetadataHooksTrait containting single function custom_uri()
• Calling custom_uri() internally by erc721_metadata_component::token_uri().
• Working implementation:
  https://github.com/rsodre/512karat/blob/main/dojo/src/systems/karat_token.cairo#L271-L287

Breaking changes

Contracts that do not implement this hook are required to import ERC721MetadataHooksEmptyImpl.

use token::components::token::erc721::erc721_metadata::erc721_metadata_component;
use token::components::token::erc721::erc721_metadata_hooks::ERC721MetadataHooksEmptyImpl;

Checklist

• Linked relevant issue
• Updated relevant documentation
• Updated relevant presets
• Added relevant tests
• Add a dedicated CI job for new examples
• Performed self-review of the code