This PR adds two new admin APIs in Minio server and madmin package:
- GetConfigKeys(keys []string) ([]byte, error)
- SetConfigKeys(params map[string]string) (err error)
A key is a path in Minio configuration file, (e.g. notify.webhook.1)
The user will always send a string value when setting it in the config file,
the API will know how to convert the value to the appropriate type. The user
is also able to set a raw json.
Before setting a new config, Minio will validate all fields and try to connect
to notification targets if available.
