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>

requirements.txt

@@ -4,15 +4,18 @@ annotated-types==0.7.0
 anyio==4.8.0
 attrs==25.1.0
 blobfile==3.0.0
+cachetools==5.5.2
 certifi==2025.1.31
 charset-normalizer==3.4.1
 click==8.1.8
 colorama==0.4.6 ; sys_platform == 'win32'
 distro==1.9.0
+durationpy==0.9
 exceptiongroup==1.2.2 ; python_full_version < '3.11'
 filelock==3.17.0
 fire==0.7.0
 fsspec==2024.12.0
+google-auth==2.38.0
 h11==0.16.0
 httpcore==1.0.9
 httpx==0.28.1
@@ -22,18 +25,22 @@ jinja2==3.1.6
 jiter==0.8.2
 jsonschema==4.23.0
 jsonschema-specifications==2024.10.1
+kubernetes==32.0.1
 llama-stack-client==0.2.2
 lxml==5.3.1
 markdown-it-py==3.0.0
 markupsafe==3.0.2
 mdurl==0.1.2
 numpy==2.2.3
+oauthlib==3.2.2
 openai==1.71.0
 packaging==24.2
 pandas==2.2.3
 pillow==11.1.0
 prompt-toolkit==3.0.50
 pyaml==25.1.0
+pyasn1==0.6.1
+pyasn1-modules==0.4.2
 pycryptodomex==3.21.0
 pydantic==2.10.6
 pydantic-core==2.27.2
@@ -45,8 +52,10 @@ pyyaml==6.0.2
 referencing==0.36.2
 regex==2024.11.6
 requests==2.32.3
+requests-oauthlib==2.0.0
 rich==13.9.4
 rpds-py==0.22.3
+rsa==4.9
 setuptools==75.8.0
 six==1.17.0
 sniffio==1.3.1
@@ -57,3 +66,4 @@ typing-extensions==4.12.2
 tzdata==2025.1
 urllib3==2.3.0
 wcwidth==0.2.13
+websocket-client==1.8.0