`coder_app` breaks when `command = "..."` and `subdomain = true`

Version: [`v0.27.1+65583ec`](https://github.com/coder/coder/commit/65583eca47f6d9d194ed8b038c3c010a4cd331c4)

If you create a `coder_app` like the following, you get a 500 error from Coder.

Either support for subdomains with commands should be added or the Terraform provider should complain that `subdomain = true` isn't allowed when `command` is set. When setting the `subdomain` key to `false` (or removing it entirely) things work fine (apart from the URL not being a subdomain).

```terraform
resource "coder_app" "keeper-commander" {
agent_id = coder_agent.main.id
slug = "keeper-commander"
display_name = "Keeper CLI"
icon = "https://www.keepersecurity.com/assets/branding/android-icon-192x192.png"
command = "keeper shell"
subdomain = true
share = "owner"
}
```