[FLINK-19133] Open custom partitioners in KafkaSerializationSchemaWrapper

[dawidwys]

What is the purpose of the change

Fix the issue that custom partitioners were not opened.

Brief change log

• Moved openning of the partitioner from initProducer to open
• Call Partitioner#open in KafkaSerializationSchemaWrapper#open

Verifying this change

Added test FlinkKafkaProducerTest#testOpenKafkaCustomPartitioner

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 e8a32ce (Fri Sep 04 06:13:47 UTC 2020)

Warnings:

• No documentation files were touched! Remember to keep the Flink docs up to date!

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Review Progress

• ❓ 1. The [description] looks good.
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• ❓ 3. Needs [attention] from.
• ❓ 4. The change fits into the overall [architecture].
• ❓ 5. Overall code [quality] is good.

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CI report:

• e8a32ce Azure: CANCELED Azure: SUCCESS

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[dawidwys]

@flinkbot run azure

[aljoscha]

The changes look good to merge once CI is green!