feat(sync): allow to bypass drop statements when sync with alter enabled

[Floppy012]

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• 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?
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Description of change

This is a less impactful version of a PR from last year (#9732 #9731). This feature implements the ability for the alter option to take an object for further configuration of the alter process during sequelize.sync(). Inside of the object the developer then can decide whether to allow drop statements or not using the drop option.

To give a small example why this feature is useful: I'm currently developing an app that can be extended by plugins. These plugins should have the ability to extend existing database models. I found out about the alter option in sync but it is currently not suitable for me since the end-user would instantly lose data if a plugin would not load correctly or would get removed by the user.

Usage of this feature would look like this:

// Alter table but don't generate drop statements
sequelize.sync({ alter: { drop : false } });

// Normal alter behavior as expected
sequelize.sync({ alter: true });

// Same behavior as { alter: true } (for safety reasons)
sequelize.sync({ alter: {} });

// Normal behavior as expected
sequelize.sync();

[codecov]

Codecov Report

Merging #11708 into master will decrease coverage by 5.95%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #11708      +/-   ##
==========================================
- Coverage   95.83%   89.88%   -5.96%     
==========================================
  Files          91       92       +1     
  Lines        8987     9095     +108     
==========================================
- Hits         8613     8175     -438     
- Misses        374      920     +546

Impacted Files	Coverage Δ
lib/sequelize.js	95.31% <ø> (ø)	⬆️
lib/model.js	96.48% <100%> (-0.07%)	⬇️
lib/dialects/mssql/query-generator.js	2.62% <0%> (-93.3%)	⬇️
lib/dialects/mssql/query-interface.js	8.69% <0%> (-91.31%)	⬇️
lib/dialects/mssql/query.js	5.58% <0%> (-89.95%)	⬇️
lib/dialects/mssql/resource-lock.js	22.22% <0%> (-77.78%)	⬇️
lib/dialects/mssql/data-types.js	30.12% <0%> (-69.88%)	⬇️
...dialects/abstract/query-generator/helpers/quote.js	93.33% <0%> (-6.67%)	⬇️
lib/dialects/abstract/query-generator.js	95.81% <0%> (-1.2%)	⬇️
... and 10 more

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[sushantdhiman]

Thanks :)