ConfigService (LTI Launch 1.4.6 API)

```
public interface ConfigService
```
This is an abstraction for configuration. It is used to look up key/value pairs of configuration items from some config source. Most of our applications currently use a simple database table but the config items could also come from an application properties file or something similar. Example config keys that might be requested from this service include things like:
- canvas_url: URL of the Canvas instance this application is bound to
- oauth_client_id: application key used to request OAuth tokens from users
- oauth_client_secret: application secret for requesting OAuth tokens from users

- Method Summary
  
  All Methods Instance Methods Abstract Methods
  Modifier and Type Method Description
  
  String getConfigValue(String key)

- Method Detail
  - getConfigValue
```
String getConfigValue(String key)
```