feat: Allow openai-key to be retrieved via a function

Enhance the handling of the openai-key variable. If it is a string it
will be interpreted as the raw key. If it is a function then that
function will be called to retrieve the key. This allows storing the
key in some secure store (e.g. auth-source)

Author: sw1nn

openai-chat.el

@@ -60,7 +60,7 @@ STREAM, STOP, MAX-TOKENS, PRESENCE-PENALTY, FREQUENCY-PENALTY, and LOGIT-BIAS."
   (openai-request "https://api.openai.com/v1/chat/completions"
     :type "POST"
     :headers `(("Content-Type"  . "application/json")
-               ("Authorization" . ,(concat "Bearer " key)))
+               ("Authorization" . ,(concat "Bearer " (openai--resolve-key key))))
     :data (openai--json-encode
            `(("model"             . ,model)
              ("messages"          . ,messages)
@@ -92,6 +92,11 @@ STREAM, STOP, MAX-TOKENS, PRESENCE-PENALTY, FREQUENCY-PENALTY, and LOGIT-BIAS."
   :type 'number
   :group 'openai)
 
+(defun openai--resolve-key (key)
+  (if (functionp key)
+      (funcall key)
+    key))
+
 ;;;###autoload
 (defun openai-chat-say ()
   "Start making a conversation to OpenAI.

openai.el

@@ -57,11 +57,36 @@
   (when openai--show-log
     (apply 'message fmt args)))
 
+(defun openai-key--auth-source ()
+  "Retrieve the OpenAI API key from auth-source."
+  (let ((auth-info (auth-source-search :max 1
+                                       :host "api.openai.com"
+                                       :require '(:user :secret))))
+    (if auth-info
+          (funcall (plist-get (car auth-info) :secret))
+	(error "OpenAI API key not found in auth-source"))))
+
 ;;
 ;;; Request
 
-(defvar openai-key ""
-  "Generated API key.")
+
+(defcustom openai-key ""
+  "Variable storing the openai key or a function to retrieve it.
+
+The function should take no arguments and return a string containing the key.
+
+A function, `openai-key--auth-source',  that retrieves the key from auth-source is provided for convenience.
+"
+  :type '(choice string function)
+  :set (lambda (option value)
+         (cond ((stringp value)
+                  (set-default option value))
+               ((functionp value)
+                  (set-default option (symbol-name value)))
+               (t
+                  (error "Invalid value for %s" option))))
+  :group 'openai)
+
 
 (defvar openai-user ""
   "A unique identifier representing your end-user, which can help OpenAI to