Make `dns_servers` a separated resource block

[lonegunmanb]

Describe your changes

Based on #167 this pr added a separated azurerm_virtual_network_dns_servers block, so the users could create link to dns servers outside, and ignore changes on the virtual network resource.

Issue number

000

Checklist before requesting a review

• [ ] The pr title can be used to describe what this pr did in CHANGELOG.md file
• [ ] I have executed pre-commit on my machine
• [ ] I have passed pr-check on my machine

Thanks for your cooperation!