Feature: source/target keys in belongsToMany associations

[swarthy]

Pull Request check-list

Please make sure to review and check all of these items:

• Does npm run test or npm run test-DIALECT pass with this change (including linting)?
• Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
• Have you added new tests to prevent regressions?
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Description of change

Closes #4092

[codecov]

Codecov Report

Merging #11311 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #11311      +/-   ##
==========================================
+ Coverage   96.35%   96.35%   +<.01%     
==========================================
  Files          94       94              
  Lines        9071     9089      +18     
==========================================
+ Hits         8740     8758      +18     
  Misses        331      331

Impacted Files	Coverage Δ
lib/dialects/abstract/query-generator.js	97.6% <100%> (-0.01%)	⬇️
lib/associations/belongs-to-many.js	98.06% <100%> (+0.13%)	⬆️

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[swarthy]

Need some help with test coverage, what I'm doing wrong 😄 ?

[papb]

LGTM! Great PR!

Thank you very much for investing some time to help Sequelize!

(Let's see what @sushantdhiman has to say!)

[papb]

Need some help with test coverage, what I'm doing wrong smile ?

I don't see anything wrong with the coverage 👍

[sushantdhiman]

🎉 This PR is included in version 5.15.0 🎉

The release is available on:

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

Your semantic-release bot 📦🚀