chore: Add stream method to LaasApiClient for streaming chat completions API response

Update LaasApiClient class to include the chat_completions method for making API calls to the chat completions endpoint. This allows users to interact with the chat completions API and receive responses. Also, remove the unused 'hash' parameter from the call_llm_preset method.

chore: Refactor LaasApiClient class for improved readability and error handling

chore: Add LaasApiClient class for making API calls to the LAAS service