[auth][client] make client accept api key via api_key parameter and PHOENIX_API_KEY env var

[axiomofjoy]

If one exists, add the api key as a header to the httpx client inside phoenix.Client so the header is automatically included in all requests.

Looks like there are several ways this is done in practice:

• Authorization: Bearer or a custom authorization scheme, e.g., Authorization: API-Key
• custom header, e.g., Arize-Phoenix-API-Key
• X-API-Key, although this format seems to be deprecated

See:

• https://platform.openai.com/docs/api-reference/chat/create
• https://docs.anthropic.com/en/api/messages