Add BulkSet as a GraphSON type

Add BulkSet as a GraphSON type. Should bring better consistency to how Gremlin works across variants and set things up better for GraphBinary format. This change has a minor "breaking" impact in that 3.4.0 will no longer coerce BulkSet to a List on the server and so a 3.3.x client that gets that type won't know what to do with it.