chore: add the --ephemeral server flag (#16126)

Another PR to address https://github.com/coder/coder/issues/15109.

Changes:
- Introduces the `--ephemeral` flag, which changes the Coder config
directory to a temporary location. The config directory is where the
built-in PostgreSQL stores its data, so using a new one results in a
deployment with a fresh state.

The `--ephemeral` flag is set to replace the `--in-memory` flag once the
in-memory database is removed.

coderd/apidoc/docs.go

@@ -10875,6 +10875,9 @@ const docTemplate = `{
                 "enable_terraform_debug_mode": {
                     "type": "boolean"
                 },
+                "ephemeral_deployment": {
+                    "type": "boolean"
+                },
                 "experiments": {
                     "type": "array",
                     "items": {