fix KafkaInputFormat when used with Sampler API

[clintropolis]

Fixes #13879.

Description

Fixes issues when using KafkaInputFormat with the sampler API. My initial assessment in #13879 was only partially correct. The implementation of the sample method in KafkaInputReader also wasn't really correct since by just using the normal read path it did not correctly capture ParseException and so would fail immediately. I've adjusted the sample method to delegate to the underlying readers sample, which will correctly handle these, and changed the blending to occur on the InputRowListPlusRawValues of the underlying sample call instead.

Release note

The KafkaInputFormat now supports using the sampler API /druid/indexer/v1/sampler.

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This PR has:

• been self-reviewed.
• a release note entry in the PR description.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• been tested in a test Druid cluster.