feat: add external contract registering in the world

[remybar]

Description

TO BE WRITTEN

Related issue

N/A

Tests

• [X] Yes
• [ ] No, because they aren't needed
• [ ] No, because I need help

Added to documentation?

• [ ] README.md
• [X] Dojo Book
• [ ] No documentation needed

Checklist

• [X] I've formatted my code (scripts/rust_fmt.sh, scripts/cairo_fmt.sh)
• [X] I've linted my code (scripts/clippy.sh, scripts/docs.sh)
• [X] I've commented my code
• [X] I've requested a review after addressing the comments