CLIENTS-2607 - Updated docs to emphasize using logger instead of log_…

[pranavrth]

These changes are related to the users getting confused for logger callback property. In the docs we refer to librdkafka properties first then explain logger properties of python client. Due to this, users tries to use log_cb property for log callback and fail.

In this PR, I tried to change our docs to reflect to use logger instead of log_cb configuration property.

Changes in the PR:

1. Updated configuration property doc to make sure that we specify python client specific properties before librdkafka properties.
2. added test cases to make sure the custom logger is logging in the correct format

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[pranavrth]

Docs would look like this:-

[pranavrth]

Merged the changes in another PR - https://github.com/confluentinc/confluent-kafka-python/pull/1447.

Closing this one.