feat: add association decorators

[ephys]

Pull Request Checklist

• Have you added new tests to prevent regressions?
• If a documentation update is necessary, have you opened a PR to the documentation repository?
• Did you update the typescript typings accordingly (if applicable)?
• Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
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Description Of Change

This is part of #15334

This PR adds decorators @BelongsTo, @HasMany, @HasOne, and @BelongsToMany.

Their signature is not identical to the one from sequelize-typescript:

• The @ForeignKey decorator was only ever used as a way for the above 4 decorators to guess which attribute was the foreign key. From experience, this ended up being more confusing than helpful. I have not included it in my implementation. Instead, users are required to specify the foreignKey parameter in @BelongsTo, @HasMany, @HasOne, and @BelongsToMany.
• Type is stricter for @BelongsTo, @HasMany, and @HasOne: All of them check that the foreign key has been defined (in type) on the proper model
• All signatures of @BelongsToMany have been dropped except one. They were all too difficult to read. The only remaining signature is @BelongsToMany(target, options)
• Pass models object in association annotations sequelize-typescript#1206 has been implemented in this PR

This is the last PR that was really essential to make the decorator system usable. The remaining 5 decorators are shortcuts