[FLINK-8141] [flip6] Fix JsonPlan serialization in JobDetailsInfo

[tillrohrmann]

What is the purpose of the change

The JsonPlan in JobDetailInfo must be serialized as a raw value
to make it parsable for downstream applications.

Verifying this change

This change is already covered by existing tests, such as (please describe tests).

• JobDetailsInfoTest

Tested the changes manually as well.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

[tillrohrmann]

Merging this PR.