This pull request introduces support for specifying an API key to authenticate requests to the VLLM server. The API key can now be passed as a CLI option (--api-key) to the relevant commands, or in config file. When provided, the API key is added to the HTTP headers in requests made to the server.

Changes:

1. Added an api_key CLI option to commands such as system-check, create, and curate. Also added into llm_client.py.
2. Modified the server check logic to include an Authorization header when an API key is provided.
3. Ensured backward compatibility by falling back to configuration settings if the CLI option is not used.

Motivation and Context:

• Enhances the security of the Synthetic Data Kit by allowing more flexible authentication mechanisms.
• Provides a straightforward way to configure API credentials both via CLI and configuration files.