fix: Add dns64 routes

[mikesplain]

Description

Based on the nat64/dns64 docs, a route is required to utilize dns64 & nat64 for IPv6 only services to non-IPv6 services.

• Add private subnet route for dns64.
• Add database subnet route for dns64.
• Enable nat gateway in dualstack example to test.

Motivation and Context

Fixes #923

Breaking Changes

How Has This Been Tested?

• [x] I have updated at least one of the examples/* to demonstrate and validate my change(s)
• [x] I have tested and validated these changes using one or more of the provided examples/* projects
• [x] I have executed pre-commit run -a on my pull request

I tested this both in our internal stack and against the ipv6-dualstack example. New route should be created as long as nat gateways are enabled other criteria you'll see in the PR.

I only updated private subnet route tables and database subnet route tables since it seemed to fit the existing patterns but if I missed anything or you feel differently, I'm open to any suggestions. Thanks!

[github-actions[bot]]

This PR has been automatically marked as stale because it has been open 30 days with no activity. Remove stale label or comment or this PR will be closed in 10 days

[mikesplainsonos]

@bryantbiggs Mind taking a look at this PR when you have a moment? Thanks!

[antonbabenko]

This PR is included in version 4.0.2 :tada:

[github-actions[bot]]

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