Accept api_key in from_provider

[jeroenvds]

Is your feature request related to a problem? Please describe.
It would be helpful if you could provide the API key as part of the from_provider constructor, as settings are often managed outside the environment variables. Currently the only option is to pass it to the client, but that removes the LLM provider-agnostic solution of from_provider()

Describe the solution you'd like
This code should be accepted

    instructor_client = instructor.from_provider(
        model=settings.LLM_MODEL,
        api_key=settings.LLM_API_KEY
    )

Today this results in an Exception:
The api_key client option must be set either by passing api_key to the client or by setting the OPENAI_API_KEY environment variable

Describe alternatives you've considered

• Environment variables: Not easy to change at runtime etc.
• Setting the API key directly on the client: Impossible to write LLM provider-agnostic code

Additional context
n/a

[Gable-github]

@jxnl i would like to work on this enhancement. Could you assign this issue to me? Thanks!

[jxnl]

@jxnl i would like to work on this enhancement. Could you assign this issue to me? Thanks!

ty!