[ECS] [request]: Custom alias for task definition revision

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**Tell us about your request**
When creating a new revision of a task definition, I want to give it a custom alias, so that I can quickly tell what it contains, at a glance, without having to inspect the JSON.

This custom alias would be displayed in the UI, within the revision name e.g.

* With alias: `my-definition:0 (foo)`
* Without alias: `my-definition:0`

One way to implement this would be to use a special tag, such as `_customAlias`. If the task definition revision contained this special tag, the alias would be visible in the UI.

Example task definition:

```
{
"family": "batman",
// ...
"tags": [
{
"key": "_customAlias",
"value": "gotham"
}
]
}
```

...which would be displayed as `batman:0 (gotham)` in the UI.

**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?**
We use CI to generate new task definitions. This process is typically associated with a git SHA. For example, I might build a docker image for the latest version of the `main` branch, and then register a new task definition with this docker image.

In this scenario, when I see `my-definition:42` in the UI, it doesn't really tell me anything. If I want to know the docker image, for example, I'm forced to inspect the task definition JSON.

With a custom alias, I could include the git SHA alongside the name e.g. `my-definition:42 (135be1)`. This provides immediate context about that specific revision (I know exactly which commit its associated with 😎).

**Are you currently working around this issue?**
The "workaround" is to inspect the task definition JSON, which is tedious... especially when managing dozens of task definition families.

**Additional context**
N/A

**Attachments**
N/A

No repository files, tests, or entry points are identified. Start by reviewing the ECS task-definition revision request and its proposed _customAlias tag approach, then determine the implementation surface for displaying an alias such as `batman:0 (gotham)` and how completion would be verified.

Written by the indexing model from the issue text.