feat: Add capability to set default for environment variable in config (#1248)

Support `${ENV_VALUE:default_value}`

🛠️ Fixes #1001

docs/en/getting-started/configure.md

@@ -22,6 +22,11 @@ etc., you could use environment variables instead with the format `${ENV_NAME}`.
   user: ${USER_NAME}
   password: ${PASSWORD}
 ```
+A default value can be specified like `${ENV_NAME:default}`.
+
+```yaml
+  port: ${DB_PORT:3306}
+```
 
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