feat(externalnetwork+gateway): vpn gateway configuration options

[roleyfoley]

Description

AWS implementation of hamlet-io/engine#1339
Uses an epilogue and cli command to apply the changes as they aren't available via cloudformation.

Motivation and Context

IPSec VPN connecitons often require specific configuration to align with network devices and security recommendation. This allows for strict control over the vpn connection options

How Has This Been Tested?

Tested on local deployment

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.