separate: true for many to many associations

[dor6-deepcred]

Issue Creation Checklist

• I have read the contribution guidelines

Feature Description

Is your feature request related to a problem? Please describe.

when including a one to many association you can pass separate: true to do this in a different query, this is great because when we have such relationship when we query serval resources it will be more performant to query their associations separately, active record (the Rails ORM) even do that by default when possible.
the problem is for some reason, in Sequelize, this is limited to one to many and excluded from many to many.

Describe the solution you'd like

I would like to add the separate: true option for many to many associations

Why should this be in Sequelize

I think its a core feature in an ORM

Describe alternatives/workarounds you've considered

doing this manually

Feature Request Checklist

Is this feature dialect-specific?

• No. This feature is relevant to Sequelize as a whole.
• Yes. This feature only applies to the following dialect(s): XXX, YYY, ZZZ

Would you be willing to implement this feature by submitting a Pull Request?

• Yes, I have the time and I know how to start.
• Yes, I have the time but I don't know how to start, I would need guidance.
• No, I don't have the time, although I believe I could do it if I had the time...
• No, I don't have the time and I wouldn't even know how to start.