[EDP] Add a Spark job type (instead of overloading Java)

Spark EDP has been implemented initially using the Java job type. However, it will be better to suport a specific Spark job type for several reasons:

* the semantics are slightly different. Spark requires a "main" application jar and supporting libs are optional (Java uses all libs)
* the Spark job type may someday support Python apps
* the possible config set for Spark will be different from Java (although they both use edp.java.main_class)
* Spark/Java may diverge in different ways in the future

This will need support in sahara-api and the dashboard, there should be no data model
or client impact.