feat: Add Note about implement external trait to external type, wrapper pattern, add example

[afifurrohman-id]

[novalagung]

I'll circle back to this PR with intensive review. at a glance there are a bunch of new phrases or keywords that need to be explained clearly, e.g. what is internal-external trait, what is local type. these keywords are not used in this ebook, adding them without a clear explanation potentially generates more confusion for the reader.

it is better to carefully keep in mind the narration because our target audience is beginner.

I appreciate the PR. Will have it reviewed and merged soon once I have the time. thanks!