Allow using custom port for webhook

[abelperezok]

Related to open-telemetry/opentelemetry-helm-charts#214

Currently there are command line flags for metrics and health checks but not for the webhook itself

	pflag.StringVar(&metricsAddr, "metrics-addr", ":8080", "The address the metric endpoint binds to.")
	flag.StringVar(&probeAddr, "health-probe-addr", ":8081", "The address the probe endpoint binds to.")

It'd be good to add another flag something like:

--webhook-port=9444

Which then would be passed to

mgrOptions := ctrl.Options{ 
    Port: //   .....  value from the flag 
}

In this line https://github.com/open-telemetry/opentelemetry-operator/blob/main/main.go#L151

[yuriolisa]

@abelperezok, does #899 close this issue?

[abelperezok]

@yuriolisa Yes, thanks.