Serialization of TinkerGraph for subgraph() step should be supported by default

[FlorianHockmann]

Describe the feature:
Serialization of a TinkerGraph should be supported by default, both by JanusGraph Server and also by the Gremlin Console that we ship. We can solve this by adding the TinkerPop IO registry in addition to our own IO registry for JanusGraph Server and in the config files included in the zip archive distribution.

Currently users get this exception if they try to use the subgraph() step without changing the configuration:

org.apache.tinkerpop.gremlin.driver.exception.ResponseException: Error during serialization: Class is not registered: org.apache.tinkerpop.gremlin.tinkergraph.structure.TinkerGraph Note: To register this class use: kryo.register(org.apache.tinkerpop.gremlin.tinkergraph.structure.TinkerGraph.class);

This issue was also reported on gremlin-users: https://groups.google.com/g/gremlin-users/c/DNcCDcYQ9fA

Describe a specific use case for the feature:
The subgraph() step can be used to get a part of the whole graph back that can then be further traversed or exported to a file.