Add API to access raw JSON data

Currently, you can not access any parameter value from the json payload, if it's not explicitely mapped in the client, for instance accessing the contextdir parameter of a build source, is impossible until the API explicitely exposes it (OSJC-203).

A way to access the raw JSON value of any given key would have helped in that case, like `Object IBuildSource.getRawProperty("context.dir")`