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>

pyproject.toml

@@ -39,6 +39,7 @@ dependencies = [
     "tiktoken",
     "pillow",
     "h11>=0.16.0",
+    "kubernetes",
 ]
 
 [project.optional-dependencies]
@@ -48,7 +49,7 @@ dev = [
     "pytest-cov",
     "pytest-html",
     "pytest-json-report",
-    "nbval",            # For notebook testing
+    "nbval",              # For notebook testing
     "black",
     "ruff",
     "types-requests",
@@ -56,7 +57,7 @@ dev = [
     "pre-commit",
     "uvicorn",
     "fastapi",
-    "ruamel.yaml",      # needed for openapi generator
+    "ruamel.yaml",        # needed for openapi generator
 ]
 # These are the dependencies required for running unit tests.
 unit = [
@@ -67,7 +68,7 @@ unit = [
     "pypdf",
     "chardet",
     "qdrant-client",
-    "opentelemetry-exporter-otlp-proto-http"
+    "opentelemetry-exporter-otlp-proto-http",
 ]
 # These are the core dependencies required for running integration tests. They are shared across all
 # providers. If a provider requires additional dependencies, please add them to your environment