Adds hf-xet license (#3116)

# Description
It appears that `isaaclab` transitively depends on `hf-xet`, (a
`huggingface`-developed data storage paradigm) via the following
dependency chain:
```
Collecting hf-xet<2.0.0,>=1.1.3 (from huggingface-hub<1.0,>=0.34.0->transformers->isaaclab==0.41.5)
```
This results in a failing license check. This PR adds the relevant
license from this [repo](https://github.com/huggingface/xet-core)

**UPDATE:** I have also added the initialization of `FAILED_PACKAGES=0`
in `license-check.yaml` in order to avoid the error I was encountering
when no packages were failing:
<img width="1461" height="210" alt="image"
src="https://github.com/user-attachments/assets/5845626e-2e5c-493f-8356-ae00667ab9ea"
/>

## Type of change

<!-- As you go through the list, delete the ones that are not
applicable. -->

- Bug fix (non-breaking change which fixes an issue)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

<!--
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putting an x character in it

For example,
- [x] I have done this task
- [ ] I have not done this task
-->

.github/workflows/license-check.yaml

@@ -56,6 +56,9 @@ jobs:
         # Load the exceptions list from the exceptions.json file
         EXCEPTIONS_FILE=".github/workflows/license-exceptions.json"
 
+        # Initialize counter for failed packages
+        FAILED_PACKAGES=0
+
         # Get the list of installed packages and their licenses
         pip-licenses --from=mixed --format=json > licenses.json
 

.github/workflows/license-exceptions.json

@@ -370,5 +370,10 @@
     "package": "anyio",
     "license": "UNKNOWN",
     "comment": "MIT"
+  },
+  {
+    "package" : "hf-xet",
+    "license" : "UNKNOWN",
+    "comment": "Apache 2.0"
   }
 ]