Add an API to get the target platform to use for a given studio project

Given a Studio project that a user has access to
The user should be able to know which target platform to use for this project
In order to select the right platform to run

Direct use is for nuxeo.io, so that the end-user doesn't have to select its target platform twice (one in studio and one in nuxeo.io).

The API should include fallback mecanism in case of a missing target platform. (if the version selected in studio doesn't fit an existing target platform for instance)