Add DNS configuration to vpn to allow it to be used to tell if Coder Connect is running.

[sreya]

We need a way for our various clients like plugins/IDE extensions and the CLI to be able to determine if they should use Coder Connect when connecting to a workspace.

For this we'll use DNS queries. DNS (unlike, say a ping or HTTP request to a well known HTTP endpoint on the Coder Connect tunnel) has the property of returning a definite result in both cases whether Coder Connect is running (AAAA record) or not (NXDOMAIN).

• Ensure DNS TTL for Coder Connect records is 2 seconds or less, to avoid cached responses giving incorrect results
• Add a generic record is.coder.connect.enabled.right.now--.<suffix> which returns the tunnel service address. This address can be used when you want to know if Coder Connect is running, but are not trying to connect to a specific known workspace.

is.coder.connect.enabled.right.now--.<suffix> has 6 label components and double-dashes which are not allowed in workspace names. This ensures it can never collide with a legitimate workspace hostname.

[sreya]

During our weekly call we talked about determining whether Desktop is running by querying DNS