feat(ec2-alpha): add Transit Gateway L2

[paulhcsun]

Issue # (if applicable)

Closes #17528

Description of changes

Create new Transit Gateway L2 with the following constructs:

• TransitGateway
• TransitGatewayRouteTable
• TransitGatewayRoute
• TransitGatewayVpcAttachment
• TransitGatewayRouteTableAssociation
• TransitGatewayRouteTablePropagation

Important Design Decision

As described in the README, the CDK disables the creation of the default route table by EC2 and instead the CDK will create a "custom" default route table in its place. This is primarily because there is no way to obtain the route table ID of the EC2 created default route table without a custom resource.

The CDK will disable the defaultRouteTablePropagation and defaultRouteTableAssociation properties on the L1 (when both are disabled, EC2 does not create the default route table), but the properties are still exposed on the CDK TransitGateway interface to allow it to be toggled for the CDK created default route table. The automatic association/propagation is being mimicked by CDK implementation and not relying on the actual EC2 feature.

Describe any new or updated permissions being added

n/a

Description of how you validated changes

Unit + Integration tests to verify that the deployed resources behave as expected.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

---

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

[aws-cdk-automation]

The pull request linter fails with the following errors:

❌ Features must contain a change to a README file.

If you believe this pull request should receive an exemption, please comment and provide a justification. A comment requesting an exemption should contain the text Exemption Request. Additionally, if clarification is needed, add Clarification Request to a comment.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.84%. Comparing base (9ae1d34) to head (4b63235).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #32956   +/-   ##
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  Coverage   80.84%   80.84%           
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  Files         232      232           
  Lines       14135    14135           
  Branches     2460     2460           
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  Hits        11428    11428           
  Misses       2427     2427           
  Partials      280      280           

Flag	Coverage Δ
suite.unit	80.84% <ø> (ø)

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Components	Coverage Δ
packages/aws-cdk	79.64% <ø> (ø)
packages/aws-cdk-lib/core	82.17% <ø> (ø)

✅ Updated pull request passes all PRLinter validations. Dismissing previous PRLinter review.

[GavinZZ]

Great work! Left some comments to help me understand.

[aws-cdk-automation]

(This review is outdated)

✅ Updated pull request passes all PRLinter validations. Dismissing previous PRLinter review.

[shikha372]

LGTM.. Thanks for all the hard work @paulhcsun !!
adding do-not-merge label for final approval from @GavinZZ

[gracelu0]

Sorry for the late review, just a few nits and questions - great work @paulhcsun ! 👏

[GavinZZ]

Thanks for the great work and patiently addressing the comments!

[gracelu0]

[paulhcsun]

@Mergifyio update

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update

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[paulhcsun]

@Mergifyio update

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update

☑️ Nothing to do

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[paulhcsun]

@Mergifyio update

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update

✅ Branch has been successfully updated

[aaythapa]

@Mergifyio update

[mergify]

update

✅ Branch has been successfully updated

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AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: e63d92a
• Result: SUCCEEDED
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