[ECS] [request]: Allow Subnets and Security Groups to be defined in the Task Definition

### Community Note

* Please vote on this issue by adding a 👍 [reaction](https://blog.github.com/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/) to the original issue to help the community and maintainers prioritize this request
* Please do not leave "+1" or "me too" comments, they generate extra noise for issue followers and do not help prioritize the request
* If you are interested in working on this issue or have submitted a pull request, please leave a comment

**Tell us about your request**

The only parameters for a task run that cannot be defined in the Task Definition are Subnets and Security Groups. Why is that? It would greatly simplify task runs from EventBridge events, and the CLI if these parameters could be part of the Task Definition.

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

ECS and ECS Fargate

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

We have tasks-definitions that are run through both EventBridge and the AWS CLI. It is a burden to keep the network configuration for these task runs consistent.

It is also laborious to type out task runs in the CLI `aws ecs run-task --network-configuration awsvpcConfiguration={subnets=[string,string],securityGroups=[string,string],assignPublicIp=string} --task-definition ...`

Start by reviewing the ECS and ECS Fargate request and the referenced `aws ecs run-task --network-configuration` command. Determine how subnets and security groups could be represented in a task definition and reused by EventBridge and CLI task runs; done means the network configuration no longer needs to be repeated for each run.

Written by the indexing model from the issue text.