[service] [request]: Fargate Spot failover to Fargate

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**Tell us about your request**
What do you want us to build?

Right now the capacityProvider strategy allows spreading tasks across Fargate and Fargate Spot launch types (and provide weightage for each type). However, if spot capacity is unavailable, the spot tasks are just not lauched, while fargate count is maintained.

There should be fall-back option, where if spot capacity is unavailable, it should fall back to using fargate launch type.

**Which service(s) is this request for?**
This could be Fargate, ECS, EKS, ECR

ECS, Fargate

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
What outcome are you trying to achieve, ultimately, and why is it hard/impossible to do right now? What is the impact of not having this problem solved? The more details you can provide, the better we'll be able to understand and solve the problem.

This is the requirement it should satisfy:

Launch 4 ECS tasks with launch type fargate spot, but if spot capacity is not available, fall back to normal fargate.

**Are you currently working around this issue?**
How are you currently solving this problem?

Not able to solve it.

**Additional context**
Anything else we should know?

**Attachments**
If you think you might have additional information that you'd like to include via an attachment, please do - we'll take a look. (Remember to remove any personally-identifiable information.)

Review the ECS capacity-provider strategy and Fargate Spot behavior described in the issue first. No repository files, tests, or implementation entry points are identified. Done means ECS can launch four tasks using Fargate Spot and fall back to normal Fargate when Spot capacity is unavailable.

Written by the indexing model from the issue text.