ECS Container Metadata utility class

It would be great to have a standardized way of accessing [ECS Container Metadata](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/container-metadata.html) in the `Amazon.Util` namespace, much like `EC2InstanceMetadata`

## Describe the Feature
A utility class that allows access to [ECS Container Metadata](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/container-metadata.html) available on configured ECS containers.

### Is your Feature Request related to a problem?
While it's easy to parse the metadata (it's a JSON file with the path declared in an environment variable), the metadata itself does not become immediately available. If this utility could provide a way to properly await for availability of the metadata, it would be quite user-friendly.

### Describe alternatives you've considered
My personal solution currently uses `Newtonsoft.JSON` to retry parsing the file until it becomes available. But it's a Unity specific implementation that uses coroutines.

## Additional Context
We run a cluster of game servers with ECS, with dynamically assigned ports. Our game servers need to be aware of their assigned ports and hosts' IPs so they can relay the connection information to a management service.

### Environment
If parse retry mechanisms are implemented, Unity's retry might need to differ.

* [ ] :wave: I may be able to implement this feature request
* [ ] :warning: This feature might incur a breaking change

---

This is a :rocket: Feature Request

Start by reading the Amazon.Util namespace and the existing EC2InstanceMetadata utility, then review the linked ECS Container Metadata documentation and the environment-variable path it describes. The issue does not name implementation files, tests, or a settled API; completion would need an agreed utility shape, metadata availability behavior, and Unity retry expectations.

Written by the indexing model from the issue text.