[AWS/ECS] Override statistic for the metrics

[draftbk]

Feature scope

AWS/ECS

Describe your suggested feature

When using alarms, looks like the Statistic is Average, could we support the override so that we could change this to the Statistic we want?

MonitoringFacade.monitorFargateApplicationLoadBalancer({
            addUnhealthyTaskCountAlarm: {
                ....
            },
            addHealthyTaskCountAlarm: {
                ....
            },
            addCpuUsageAlarm: {
               ....
            },
            addMemoryUsageAlarm: {
               ....
            },
        });

[koennjb]

+1! This would also be a very helpful feature for other resource types besides ECS/Fargate.