Sync updates from stainless branch: main

[vladimirivic]

What does this PR do?

We are adding api_key arg so we can use LlamaStackClient like this

from llama_stack_client import LlamaStackClient

client = LlamaStackClient(
    base_url='https://api.my-llamastack-domain.com',
    api_key = 'MY-API-KEY',
)

[vladimirivic]

Manually tested using api-key, configured client and called llama-stack-client models list