[ECS] [Feature]: Metadata Endpoint V4 to add subnet ID

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**Tell us about your request**
The ECS Metadata does not surface the Subnet ID the container is running into. We would like it to.
SImilar to #832 , we are missing this information in order to be able to determine without having to reverse engineer, which subnet our container is into. Presently on the AZ is surfaced.

**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?**
Want to be able to have our application AWS Network aware by using the ECS Metadata. Right now the only way around it is to create a service per zone with the same task definition, almost, as you'd need to change labels or tags per AZ/Subnet.

The subnet ID should be surfaced at the same level as the VPC ID in the metadata, preferably, or be added to the networks definition

See https://docs.aws.amazon.com/AmazonECS/latest/developerguide/task-metadata-endpoint-v4.html

**Are you currently working around this issue?**
Have to create multiple services.

Start with the ECS Task Metadata Endpoint V4 documentation linked in the issue and compare its current network metadata with the requested VPC-level fields. Done means the endpoint exposes the container task's subnet ID, either alongside the VPC ID or within the networks definition; no repository files or tests are identified.

Written by the indexing model from the issue text.