[ECS] [Architecture Fallback]: For fargate, allow to specify prefered architecture, fallback to another

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## The request

At the moment, when creating a new Service, I can specify the architecture I would like to use for Fargate via [RuntimePlatform.CpuArchitecture](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ecs-taskdefinition-runtimeplatform.html).

It would be great to able to define a priority list (i.e. `ARM64` first, `X86_64` second), similar to how one defines Capacity providers with base and weights.

Yes, users will need to make sure to use image manifest list and support more than one architecture for a given image.

This is for ECS + Fargate

### Why?

It happens that ECS Fargate does not have the capacity for `ARM64` and therefore the service never gets deployed. When that occurs, it would be great to have the service fallback then to using `X86_64` Fargate nodes.

### Workaround
I can't, I have to change the TaskDefinition to remove that parameter.

Thank you for your consideration,

No repository files, tests, or implementation entry points are identified in this roadmap request. Start by reviewing ECS RuntimePlatform.CpuArchitecture and Fargate capacity behavior; done would mean defining and implementing a priority list such as ARM64 followed by X86_64 with fallback when the preferred architecture lacks capacity.

Written by the indexing model from the issue text.