feat: Add Kubernetes auth provider to use SelfSubjectReview and kubernetes api server (#2559)

# What does this PR do?
Add Kubernetes authentication provider support
- Add KubernetesAuthProvider class for token validation using Kubernetes
SelfSubjectReview API
- Add KubernetesAuthProviderConfig with configurable API server URL, TLS
settings, and claims mapping
- Implement authentication via POST requests to
/apis/authentication.k8s.io/v1/selfsubjectreviews endpoint
- Add support for parsing Kubernetes SelfSubjectReview response format
to extract user information
- Add KUBERNETES provider type to AuthProviderType enum
- Update create_auth_provider factory function to handle 'kubernetes'
provider type
- Add comprehensive unit tests for KubernetesAuthProvider functionality
- Add documentation with configuration examples and usage instructions

The provider validates tokens by sending SelfSubjectReview requests to
the Kubernetes API server and extracts user information from the
userInfo structure in the response.


<!-- If resolving an issue, uncomment and update the line below -->
<!-- Closes #[issue-number] -->

## Test Plan
<!-- Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.* -->
What This Verifies:
Authentication header validation
Token validation with Kubernetes SelfSubjectReview and kubernetes server
API endpoint
Error handling for invalid tokens and HTTP errors
Request payload structure and headers

```
python -m pytest tests/unit/server/test_auth.py -k "kubernetes" -v
```

Signed-off-by: Akram Ben Aissi <akram.benaissi@gmail.com>

llama_stack/apis/common/errors.py

@@ -79,3 +79,10 @@ class ConflictError(ValueError):
 
     def __init__(self, message: str) -> None:
         super().__init__(message)
+
+
+class TokenValidationError(ValueError):
+    """raised when token validation fails during authentication"""
+
+    def __init__(self, message: str) -> None:
+        super().__init__(message)