feat(aiops) Add AIOps L2 construct for investigation group

[amandaleeatwork]

Issue # (if applicable)

aws/aws-cdk-rfcs#792.

Reason for this change

Introduce L2 construct for AIOps, aka cloudwatch investigation.

Description of changes

• What code changes did you make?
• Added new investigation L2 construct, and methods to setup cross account configuration and channel notification.
• Why do these changes address the issue?
• Creating investigation group resource would require few related resources created, iam role, permission on role, cross account configuration setup and etc. But introducing L2 construct would only expose few properties for customer's input, others would be handled underneath.
• What alternatives did you consider and reject?
• internal L2 package, but based on CDK team recommendation, we would choose the native package aws-cdk to release L2 construct
• What design decisions have you made?
• RFC: RFC 792: L2 Construct for AIOps aws-cdk-rfcs#796

Describe any new or updated permissions being added

This is brand new L2 construct, no new permissions introduced.

Description of how you validated changes

[X] Unit tests
[] Integration test

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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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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