[FLINK-38353][spans] FLIP-483: Add support for children Spans

[pnowojski]

What is the purpose of the change

https://cwiki.apache.org/confluence/display/FLINK/FLIP-483%3A+Add+support+for+children+Spans

Brief change log

Please check individual commit messages.

Verifying this change

Adds a couple of unit tests and extends the existing ITCase

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

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

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

[flinkbot]

CI report:

• 5ae8b55 Azure: FAILURE

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