[ECS] : Capacity Strategy to Fall back to OD only When No More Spot Capacity Available

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**Tell us about your request**
Ability to create a capacity strategy that allows you to use spot instances as long as the spot capacity is available, and fall back to on-demand instances only when there is no capacity available for spot.

**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?**
I was hoping that "Base" in a capacity strategy will be more of a "strategy" but it seems to be a "constraint". In my use case, I was hoping to use 5 (which is also the total number of tasks in my service) as base for my capprovider1 which entirely consists of spot instances and use a 1:1 weight. So, the base will be met as long as there are spot instance available, otherwise I was hoping it to ignore the base and fall back to the capprovider2 which has OD instances. But even when capprovider2 has instances, service fails to place tasks because it's trying to satisfy base.

**Are you currently working around this issue?**
Using lambda
Please let me know if more information is required or in case there is a better alternative.

Start by reviewing the ECS capacity provider strategy behavior described here, focusing on how the base value affects placement when the Spot-only provider has no capacity. Compare the requested fallback with the current Lambda workaround; done means tasks use Spot while available and fall back to the on-demand provider when Spot capacity is unavailable.

Written by the indexing model from the issue text.