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.

docs/reference/api/general.md

@@ -224,6 +224,7 @@ curl -X GET http://coder-server:8080/api/v2/deployment/config \
       "user": {}
     },
     "enable_terraform_debug_mode": true,
+    "ephemeral_deployment": true,
     "experiments": [
       "string"
     ],