added pubip and subnet support

[jchancellor-ms]

Description

This PR adds the ability to configure and associate individual public IP addresses to a NAT Gateway. It also enables an interface to define subnet associations for the NAT gateway.

Fixes #8 Fixes #20 Fixes #74 Closes #8 Closes #20 Closes #74

Type of Change

• [x] Non-module change (e.g. CI/CD, documentation, etc.)
• [ ] Azure Verified Module updates:
  • [ ] Bugfix containing backwards compatible bug fixes, and I have NOT bumped the MAJOR or MINOR version in locals.version.tf.json:
  • [x] Someone has opened a bug report issue, and I have included "Closes #{bug_report_issue_number}" in the PR description.
  • [ ] The bug was found by the module author, and no one has opened an issue to report it yet.
  • [x] Feature update backwards compatible feature updates, and I have bumped the MINOR version in locals.version.tf.json.
  • [ ] Breaking changes and I have bumped the MAJOR version in locals.version.tf.json.
  • [x] Update to documentation

Checklist

• [x] I'm sure there are no other open Pull Requests for the same update/change
• [ ] My corresponding pipelines / checks run clean and green without any errors or warnings
• [ ] I did run all pre-commit checks

[jchancellor-ms]

@mbilalamjad - This PR closes the two open issues for NAT Gateway. Please ping me if you have questions. It adds public IP and subnet association interfaces to implement the most common configuration for NAT gateway.

[joshbsemperis]

Can this be merged?