Add usePushEach flag to Mongoose Schema Options

[EtienneBoutet]

In order for devs to be able to work around the issues described in
Automattic/mongoose#5574 we need to be able
to supply the usePushEach flag to the Mongoose SchemaOptions. This
options functionality is described in
Automattic/mongoose#4455 and is required for
anyone who modifies arrays inline who is using Mongoose 4 and MongoDB
3.6+

Testing:
Copied changes to local project and verified compilation worked.

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[typescript-bot]

@EtienneBoutet Thank you for submitting this PR!

🔔 @horiuchi @sindrenm @lukasz-zak @Alorel @jendrikw @ethanresnick @vologab - please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

If no reviewer appears after a week, a DefinitelyTyped maintainer will review the PR instead.

[typescript-bot]

After 5 days, no one has reviewed the PR 😞. A maintainer will be reviewing the PR in the next few days and will either merge it or request revisions. Thank you for your patience!

[jendrikw]

Can you please add a test that sets this option to a boolean?