Added driver map getter

We can think of several situations where a read access to the supported drivers is required. I personnaly need it in the following use cases : 

- Validating a driver choice in a mutli-platform application installer
- Generating migration classes for all the available platforms

This PR just adds a getter for that list. As it is a harmless and non-BC-break change, it would be great if it could be merged also into the 2.3 version.