[8.x](backport #41824) Fix Kafka output panic at startup

[mergify]

• Closes [8.17] Kafka output panics unconditionally at startup #41823

In #40794 the Connectable interface was changed to include a context.Context argument, causing all clients with a Connect() that weren't updated to have a Context(context.Context) method instead to fail the NetworkClient interface check in

beats/libbeat/publisher/pipeline/client_worker.go

Lines 64 to 70 in 367d94e

	if nc, ok := client.(outputs.NetworkClient); ok {
	c = &netClientWorker{
	worker: w,
	client: nc,
	logger: logger,
	tracer: tracer,
	}

This made it so that the Kafka and Redis outputs no longer counted as NetworkClients, bypassing the initial call to Connect() that was there previously at

beats/libbeat/publisher/pipeline/client_worker.go

Line 143 in 367d94e

err := w.client.Connect(ctx)

In the case of Kafka the Connect call never happening makes the output panic on the first call to Publish because there is no producer.

Raising without tests to make sure nothing else was missed while I figure out the best test for this that isn't totally contrived, because the problem is in the publisher pipeline and the output specific tests don't hook in at that level. This is how the problem was originally missed.

The PR has been updated with an integration test to reproduce the problem: #41824 (comment)

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This is an automatic backport of pull request #41824 done by [Mergify](https://mergify.com).

[botelastic]

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