[feature request]: Cross-Account EFS mounts in Fargate and ECS/EC2

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**Tell us about your request**
Allow cross-account mounting of EFS volumes in Fargate

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
We have EFS mounts working in Fargate in the same account, however we'd like to mount a volume from another account. We have this working with a peered VPC with EC2, however it appears there is no way to fix the DNS for the EFS mount in Fargate.

**Are you currently working around this issue?**
We're hoping to use Fargate, however if this is an issue we may have to use ECS and take care of the mounts ourselves until it is available.

**Additional context**
I believe a fix for #886 would take care of this as we could specify the DNS issue ourselves as is recommended for cross-account EFS mounts.

Review issue #886 first, then investigate the Fargate/ECS EFS mounting path and AWS requirements for cross-account mounts. Confirm whether the DNS configuration requested in #886 addresses this case; done means supported cross-account EFS mounting from Fargate with the expected behavior documented.

Written by the indexing model from the issue text.