feat(aws-rds): add ability to specify PreferredMaintenanceWindow to RDS cluster database instances

[rjjaisim-at-amazon]

Issue # (if applicable)

Closes #16954

Reason for this change

Noticed that we were able to specify preferredMaintenanceWindow for a cluster, but unable to do so for the instances created under the cluster. Instead, AWS (semi-)randomly assigns a maintenance window (doc) for the instances, which leads to things being out of sync b/w the cluster and its child instances

There are some workarounds as mentioned in the issue above, but those are a little hacky (imo) and I figured adding the preferredMaintenanceWindow as an instance prop is a better long-term solution. Also, it might be hard for other developers to find the workarounds as they are only mentioned in the above issue and aren't available through normal channels (Stack overflow/official CDK docs)

Description of changes

Added optional preferredMaintenanceWindow field under InstanceProps, and passed that field in during the creation of the CfnDBInstance. Also added a quick unit test

Description of how you validated changes

Added a unit test, did not add integ tests. Ran yarn build and yarn test

Callout: I was unable to run integration tests locally, kept getting errors with yarn integ --directory packages/aws-cdk-lib/aws-rds and yarn integ-runner --directory packages/aws-cdk-lib/aws-rds - Error: Cannot find module './integ-runner.js', not sure if I'm missing something

Checklist

• [ x ] My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

No breaking changes

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

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[aws-cdk-automation]

Your pull request must be based off of a branch in a personal account (not an organization owned account, and not the main branch). You must also have the setting enabled that allows the CDK team to push changes to your branch (this setting is enabled by default for personal accounts, and cannot be enabled for organization owned accounts). The reason for this is that our automation needs to synchronize your branch with our main after it has been approved, and we cannot do that if we cannot push to your branch.

[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 415790a
• Result: SUCCEEDED
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[mergify]

⚠️ The sha of the head commit of this PR conflicts with #29033. Mergify cannot evaluate rules on this PR. ⚠️