Add support for vpn_sites and vpn_gateway_connections

[carldjohnston]

Issue-id

PR Checklist

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• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• My code follows the code style of this project.
• I ran lint checks locally prior to submission.
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Description

Adds support for vpn_sites and vpn_gateway_connections contained in aztfmod/terraform-azurerm-caf#518

Does this introduce a breaking change

• YES
• NO

Testing