feat(externalnetwork): vpn attachment lookup

[roleyfoley]

Description

Adds a lookup for the TransitGateway attachment when using a VPN connection.
Required for: hamlet-io/engine-plugin-aws#48

Motivation and Context

The VPNConnection Cloudformation resource is used to attach to a TransitGateway, as a result of this the Transit Gateway attachment Id isn't available from Cloudformation. The attachment Id is required for routing configuration and propogation config

This is covered as a request to the CFN team in aws-cloudformation/cloudformation-coverage-roadmap#308

How Has This Been Tested?

Tested locally

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Refactor (non-breaking change which improves the structure or operation of the implementation)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Followup Actions

• None

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• None of the above.