kgo: add ctx field to client cfg

[o-mago]

Adds ctx field to allow custom contexts to be used by the client
For example, it allows the custom onAssigned func, added using OnPartitionsAssigned opt, to access a custom context and any propagated value inside (like tracers)

[o-mago]

could you review it @twmb 😊?

[twmb]

Looks good, can you update the Client.OptValue method to return this context?

[o-mago]

done @twmb

[twmb]

I'm slowly working on 1.19, I plan to merge this once gearing up to release.

[twmb]

Is it worth it to add a Context() context.Context function to the client, so that the client can return the internal context (which will be this context, wrapped with a cancellation)?