Add SSL initialization callback and unit test

[cschwarz73]

Add the option to initialize the SSL context via a callback, which can be registered using a new ssl.context.init_cb configuration property. The purpose is to have more control over how to initialize SSL than what the existing configuration properties allow. This makes it easier to integrate librdkafka into applications that have their own security frameworks, by delegating the initialization job to the existing framework.

[cla-assistant[bot]]

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