Use kryo instead of java serialization for the `Reference` used in Spark

[tomekl007]

As a follow-up after: https://github.com/projectnessie/nessie/pull/3561#pullrequestreview-905457313 We should try to use kryo serialization mechanism.

Steps to do it:

1. Revert whole #3561
2. remove Serializable from Reference.java
3. run the testcase AbstractRestGCTest , it will fail with serializable exception.
4. explore using Kryo or a custom serializer for Reference.

[snazy]

@tomekl007 do we still need this ticket?