Deprecate `api_key` arguments

[hadley]

In favour of credentials callback. Only called at request time, and can be overridden by optional property.

[dylanpieper]

I'm curious if credentials will fix the redacted key problem I'm having. I can no longer clone a chat using future because of the serialization involved to spawn workers. Can it call back the env keys?

[hadley]

@dylanpieper yes, that's the plan. However, you should be able to work around the problem today, just by serializing the turns + tools and recreating chat the object in each worker.

[shikokuchuo]

A minimal solution could be for the base_request methods to attempt to re-initialize provider@api_key if it is NULL.

[hadley]

Work needed:

• Add a new credentials property to every Provider. It should be a function.
• Add a credentials argument to every chat_ function.
  • It should default to NULL.
  • It should check that the input is a zero-argument function (or NULL) with check_function2
  • If not supplied it should get (with %||%) an anonymous function that calls key_get() (using the same value as api_key)
• Update uses of @api_key to instead call @crendentals().
• Deprecate api_key argument for every provider. The default value should become NULL, and when NULL need to call lifecycle::deprecate("0.4.0", "thisname(api_key)", "thisfunction(credentials)), and wrap the value in an anonymous function.