ISSUE-443: Add context param in interceptor

[sijie]

Original Issue: apache#443

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Is your feature request related to a problem? Please describe.
I want to add opentracing tracking for message delivery. Executing opentracing related code in the interceptor is an elegant way which won't cause a bad influence on my logic code. However, the interceptor has no context parameter, which makes it difficult to pass in the relevant context.

Describe the solution you'd like
I want to add context param in ProducerInterceptor's BeforeSend method.
BeforeSend(producer Producer, message *ProducerMessage)
↓
BeforeSend(ctx context.Context, producer Producer, message *ProducerMessage)

[GPrabhudas]

As the issue points out, Is it possible to add context.Context as parameter to BeforeSend method ?

I've requirement to add some common properties to the message before sending it. So I think BeforeSend method is a good place to do this if it accepts context.Context parameter.