fix(batch): make instanceType optional on multinode jobs

[comcalvi]

Make instanceType optional. It used to be required. It will default to a batch optimal instance type.

The breaking change is for MultiNodeJobDefinition.instanceType, which is now optional. This is not truly breaking because everyone has this property explicitly set now, and will continue to have it set after this change.

Closes #27185.

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[TheRealAmazonKendra]

Weird that setting something to optional is considered a breaking change.

[rix0rrr]

On the question of making properties optional, this comes up way too often, so I made an explainer for it:

https://github.com/aws/jsii/pull/4274/files?short_path=f5d91d1#diff-f5d91d104bff5f6eba3495d9aa4ef6d0e2854a010d6c2754acdd98536afc3c05

[comcalvi]

thanks for writeup @rix0rrr, that's helpful

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[aws-cdk-automation]

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[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).