feat(aws-kms): allow tagging kms keys

[rsmogura]

Small change to enable tagging KMS keys.

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Pull Request Checklist

• [X ] Testing
  • Unit test added
  • CLI change?: manually run integration tests and paste output as a PR comment
  • cdk-init template change?: coordinated update of integration tests with team
• [X ] Docs
  • jsdocs: All public APIs documented
  • README: README and/or documentation topic updated
• [X ] Title and Description
  • Change type: title prefixed with fix, feat will appear in changelog
  • Title: use lower-case and doesn't end with a period
  • Breaking?: last paragraph: "BREAKING CHANGE: <describe what changed + link for details>"
  • Issues: Indicate issues fixed via: "Fixes #xxx" or "Closes #xxx"
• Sensitive Modules (requires 2 PR approvers)
  • IAM Policy Document (in @aws-cdk/aws-iam)
  • EC2 Security Groups and ACLs (in @aws-cdk/aws-ec2)
  • Grant APIs (only if not based on official documentation with a reference)

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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.