feat: Support providing API key from an environment variable

[siketyan]

This pull request proposes to add support of NEXTDNS_API_KEY environment variable for providing the API key to the provider. We can now run Terraform with NEXTDNS_API_KEY=xxx terraform apply without writing provider "nextdns" {} block. This is useful especially when running this provider on CI.

Note that the environment variable will be preferred over provider.nextdns.api_key.

[amalucelli]

Thanks for the improvement!

[siketyan]

Thank you for your quick review and the nice provider!