[FLINK-20175][bp-1.11] Avro Confluent Registry SQL format does not support adding nullable columns

[dawidwys]

What is the purpose of the change

• Backports the fix for nullable columns in Avro format. efc12ca
• Backports the nested record names f570d04

Verifying this change

Added test: testAddingOptionalField

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/Mesos, 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)

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Automated Checks

Last check on commit ab64e85 (Fri Nov 27 09:43:44 UTC 2020)

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[twalthr]

One minor comment otherwise good to go.

[twalthr]

Use DataType directly. We should not advertise TableSchema anymore.

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CI report:

• 70cf3a3 Azure: SUCCESS

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