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>

tests/unit/registry/test_registry_acl.py

@@ -12,7 +12,7 @@ import pytest
 
 from llama_stack.apis.models import ModelType
 from llama_stack.distribution.datatypes import ModelWithACL
-from llama_stack.distribution.server.auth import AccessAttributes
+from llama_stack.distribution.server.auth_providers import AccessAttributes
 from llama_stack.distribution.store.registry import CachedDiskDistributionRegistry
 from llama_stack.providers.utils.kvstore.config import SqliteKVStoreConfig
 from llama_stack.providers.utils.kvstore.sqlite import SqliteKVStoreImpl