feature: basic support for context

[aybabtme]

👋 I'm just messing around here to try and find a way to have contexts supported by code that I work with that uses sarama. Is this something that would be of interest? If so, I can look at writing tests and iterating and so on.

The idea being that you can set Context on a message and it will be best-effort supported by the producers. In the sync producer case, this makes a lot of mostly obvious sense: "I'm willing to wait this long to send a message, otherwise give up".

In the async producer case, the use case is a bit less obvious, since the assumption is that a message will be sent sometime in the future. Yet, since the ProducerMessage.Context is a field and not a method argument, and therefor opt-in, messages that don't set a context wouldn't see any difference. A user of the async producer would have to set the field to signify that they want to have the message sort-of respect the ctx.Done() lifetime. Why would someone do this? Perhaps they're willing to send a message async for up to 5s, after which the message would be untimely and its usefulness gone.

[storm-blue]

I need this!

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