Support inject source step in Gremlin.Net

GraphTraversalSource has no Inject step in Gremlin.Net because it's explicitly excluded in the GLV generation:

"sourceSpawnMethods": GraphTraversalSource.getMethods(). // SPAWN STEPS
                        findAll { GraphTraversal.class.equals(it.returnType) && !it.name.equals('inject')}.
// ...

I don't know why inject is explicitly excluded here, but there should be an implementation in GraphTraversalSource as the other GLVs also have this step.