[ECS] [request]: Ability to provide custom number of datapoints to alarm of target tracking alarms on ASG capacity provider creation

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**Tell us about your request**
Please, add an optional ability to provide custom number of datapoints to alarm to target tracking alarms (AlarmHigh, AlarmLow) on ASG capacity provider creation.

**Which service(s) is this request for?**
ECS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Currently new ASG capacity provider defaults AlarmHigh and AlarmLow at 1 datapoint within a minute and 15 datapoints within 15 minutes respectively. It would be nice to set our own datapoints to alarm on new capacity provider creation, especially for testing purposes.

**Are you currently working around this issue?**
We can edit those alarms in CloudWatch directly after capacity provider creation, but is it safe to do so?

No implementation files or tests are named. Start by locating the ECS capacity provider creation path and its target-tracking alarms, then trace how AlarmHigh and AlarmLow datapoints are configured. Done means an optional custom datapoint setting is accepted during ASG capacity provider creation and covered by tests.

Written by the indexing model from the issue text.