feat(types): make Model.init aware of pre-configured foreign keys

[ephys]

This PR is for Sequelize v7, #14370 is v6

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• Have you added new tests to prevent regressions?
• Does yarn test or yarn test-DIALECT pass with this change (including linting)?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Did you update the typescript typings accordingly (if applicable)?
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Description Of Change

Closes #14125

Model.init currently requires a configuration for every declared attributes.

Some attributes don't need to be configured by Model.init, these include:

• Timestamp attributes.
• Foreign keys added by association methods (like Project.belongsTo(User)).

I've resolved this by adding a new ForeignKey<> branded type (we're now at 3 branded types).
This type tells Model.init that the attribute doesn't need to be included.

I didn't include a solution for timestamp attributes, because the 'workaround' (which has been documented) is as simple and clearer than adding another branded type.

---

I've also included some documentation updates from #14091 that I want to cherry-pick in v6.

[WikiRik]

Not knowledgeable enough to actually review the changes in src/model.d.ts but in general looks good. I am wondering however if we can't add some additional tests for this (apart from the one you've already added)

[ephys]

@WikiRik resolving the conflict dismissed your review 😅

[ephys]

Thank you!

[github-actions]

🎉 This PR is included in version 7.0.0-alpha.11 🎉

The release is available on:

• npm package (@release-v7 dist-tag)
• GitHub release

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