Avoid unclosed consumer and producer warnings when using AsyncContextManager interface

[calgray]

Changes

Fixes #1130 where AIOKafkaProducer and AIOKafkaConsumer raise unclosed resource warning due to an exception raising during __aenter__(). __aenter__() when an exception is raised should cleanup any partially acquired resources.

To best reproduce the issue, regression tests via mocks have been added for all the operations that need resource cleanup if an exception appears later on in the start() method. Rather than nested try-catch blocks, AsyncExitStack handles this exact situation.

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
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