Support inline env variable substitution in rivet config

 Modify the config loader in src/config/mod.rs to support inline environment variable substitution in TOML config files (e.g., password = "${DB_PASSWORD}").


**Expected Behavior**

  Config file:
```
  [catalog]
  type = "postgres"
  password = "${DB_PASSWORD}"
```

  With DB_PASSWORD=secret in environment → password resolves to "secret"