Deserialize StateBackend from JobCheckpointingSettings with user classloader

A user ran into the problem that the SubmitJob message is not (de)serialisable if it contains custom RocksDB options (or a custom state backend): [1]

The problem is that SubmitJob contains a JobGraph which contains JobCheckpointingSettings which contains a StateBackend. This StateBackend potentially has user code and therefore can only be deserialised with the user classloader.

This issue is mostly identical to FLINK-6531.

[1] https://lists.apache.org/thread.html/69bb573787258ab34c3dc56ac155052f099d75e62d805f463bde5621@%3Cuser.flink.apache.org%3E