[ECS] [request]: Support hostname with awsvpc network mode

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**Tell us about your request**
As per [documentation for ContainerDefinition](https://docs.aws.amazon.com/AmazonECS/latest/APIReference/API_ContainerDefinition.html) I see note `The hostname parameter is not supported if you are using the awsvpc network mode.`. But I'd like to configure hostname for containers (i.e. mongos, rabbitmq, etc.) with awsvpc network mode

**Which service(s) is this request for?**
ECS (EC2 or Fargate)

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Right now we're using mongos router container as sidecar, and after migration to awsvpc network mode we're facing a lot of instances there. Previously we used bridge network and configured hostname for mongos container, therefore there was 1 instance in Mongo Cloud Manager. After a while we have a lot of instances there and page starts freezing, so we should manually delete all dropped (terminated) instances. We'd like to fix or at least automate it somehow.

**Are you currently working around this issue?**
Yes, on Dev env (we haven't migrated network mode from `bridge` to `awsvpc` on Prod) we're facing this issue.

Start with the linked AWS ContainerDefinition documentation and the ECS awsvpc network-mode limitation described in the request. Determine whether hostname support is feasible for ECS on EC2 and Fargate; done means users can configure a container hostname in awsvpc mode without the current limitation or operational workaround.

Written by the indexing model from the issue text.