[FLINK-8990] [test] Extend Kafka end-to-end test to verify partition discovery

[tzulitai]

What is the purpose of the change

This PR is based on #5778.

The changes further extends the Kafka end-to-end tests to cover verifying the partition discovery feature.

Brief change log

The whole test now does the following:

• Start of with 1 partition for the test topic
• Writes some messages, and then verify output
• Add a new partition to the test topic
• Verify that a new partition is really added to the topic
• Writes some more messages
• Verify that we really do have some messages in the new partition
• Verify new output

Verifying this change

This PR is an extension to existing tests.

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, 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)

[aljoscha]

The changes look good to merge! 👍

[tzulitai]

Thanks for the review @aljoscha.
Merging this ...