[ECS] [request]: Register container instance to the cluster in different AWS account

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**Tell us about your request**
We need to register container instance in one AWS account to a cluster in another AWS account

**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?**
We have multiple AWS accounts in an AWS organization which belongs to entire group companies. Out of these accounts, we have only selected accounts which belong to our child company. We have purchased compute savings plan in one account and we want to share that with another AWS account which belongs only to our child company under AWS organization. We are not able to do so at Organization hence we want to launch container instance in AWS account who has purchased savings plan and register these instances in ECS clusters hosted in another AWS account.

**Are you currently working around this issue?**
We do not have any option. We have to run instances in different accounts

**Additional context**
We had discussed this with AWS on how can we share savings plan and reservation only with our accounts in an organization but we could not achieve this. We have a reseller account and in that case we have to create separate payer account and link our accounts with that payer account.

**Attachments**
None

The request concerns registering an ECS container instance in one AWS account with a cluster in another, motivated by sharing compute savings plans across accounts. No files, tests, or implementation entry points are identified; first clarify the cross-account ECS requirements and define the supported configuration and completion criteria with maintainers.

Written by the indexing model from the issue text.