feat(DirectLink): allow the Directlink gateway to be attached to transit gateway

[ajay-malhotra1]

PR summary

Add connection_mode to Create/Update/Actions api for direct link gateways to specify the type of service this gateway attaches to. Mode transit means this Gateway will be attached to Transit Gateway Service and direct means this Gateway will be attached to VPC or classic connection

Fixes: NA

PR Checklist

Please make sure that your PR fulfills the following requirements:

• [x] The commit message follows the Angular Commit Message Guidelines.
• [x] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

PR Type

• [ ] Bugfix
• [x] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [x] New tests
• [ ] Build/CI related changes
• [ ] Documentation content changes
• [ ] Other (please describe)

What is the current behavior?

What is the new behavior?

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information

[ajay-malhotra1]

:tada: This PR is included in version 0.20.0 :tada:

The release is available on GitHub release

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