[ECS] [request]: Reduce HealthCheckCustomConfig.FailureThreshold

**Tell us about your request**
Add option to reduce the duration used by ECS `HealthCheckCustomConfig.FailureThreshold`, for updating Cloud Map (service discovery) registries. The [current fixed value is 30 seconds](https://docs.aws.amazon.com/cloud-map/latest/api/API_HealthCheckCustomConfig.html), and `FailureThreshold` is a multiplier for that fixed interval. The interval is too long.

**Which service(s) is this request for?**
ECS, Cloud Map (Service Discovery)

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
The lowest configurable value of `FailureThreshold=1` means having a 1 x 30 seconds interval between Cloud Map receiving an ECS health check state change, and Cloud Map acting upon that change, updating the DNS records in the namespace.

In practice, this makes Cloud Map slow to react to changes in ECS task availability.

I would like the interval to be configurable. Perhaps not down to the second, but it would be good if it could be optionally reduced to something like 2 or 5 seconds. I appreciate the fixed 30-second interval having a dampening effect, and ultimately a protection from inadvertently shooting myself in the foot, but it's not 1997 anymore and I think having the option of lowering the interval is reasonable.

**Are you currently working around this issue?**
I'm not currently working around this issue, and rather accepting the 30+ seconds it takes for Cloud Map top be updated.