feat: Add Kubernetes authentication (#1778)

# What does this PR do?

This commit adds a new authentication system to the Llama Stack server
with support for Kubernetes and custom authentication providers. Key
changes include:

- Implemented KubernetesAuthProvider for validating Kubernetes service
account tokens
- Implemented CustomAuthProvider for validating tokens against external
endpoints - this is the same code that was already present.
- Added test for Kubernetes
- Updated server configuration to support authentication settings
- Added documentation for authentication configuration and usage

The authentication system supports:
- Bearer token validation
- Kubernetes service account token validation
- Custom authentication endpoints

## Test Plan

Setup a Kube cluster using Kind or Minikube.

Run a server with:

```
server:
  port: 8321
  auth:
    provider_type: kubernetes
    config:
      api_server_url: http://url
      ca_cert_path: path/to/cert (optional)
```

Run:

```
curl -s -L -H "Authorization: Bearer $(kubectl create token my-user)" http://127.0.0.1:8321/v1/providers
```

Or replace "my-user" with your service account.

Signed-off-by: Sébastien Han <seb@redhat.com>

llama_stack/distribution/server/server.py

@@ -419,9 +419,9 @@ def main(args: Optional[argparse.Namespace] = None):
         app.add_middleware(ClientVersionMiddleware)
 
     # Add authentication middleware if configured
-    if config.server.auth and config.server.auth.endpoint:
-        logger.info(f"Enabling authentication with endpoint: {config.server.auth.endpoint}")
-        app.add_middleware(AuthenticationMiddleware, auth_endpoint=config.server.auth.endpoint)
+    if config.server.auth:
+        logger.info(f"Enabling authentication with provider: {config.server.auth.provider_type.value}")
+        app.add_middleware(AuthenticationMiddleware, auth_config=config.server.auth)
 
     try:
         impls = asyncio.run(construct_stack(config))