add waitgroup in ReadFromEgress function for AgentFinalStreamHandler

[devinyf]

The ReadFromEgress function is inside a goroutine already And in some case, It's need to wait until complete from the caller So just return a waitGroup pointer that user can choose if need to use wg.Wait() or not.

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[devinyf]

update:
found that the old implementation in for xxx := range handler.egress will never end... And will cause goroutine leak fixed this problem using HandleAgentFinish method

go func() {
		defer close(handler.egress)
		for data := range handler.egress {
			callback(ctx, data)
}()