Search auth-source for api key before authn (#110)

* search auth-sources for api-key before authn

This commit closes https://linear.app/mercury/issue/84

**Why:**
For users who have saved their api-key in an auth-source, this commit
prevents them from seeing the prompt to authenticate manually or
automatically, which can be annoying and takes time.

**How:**
implement codeium-get-saved-api-key using hardcoded values for :host and
for :user. I'm not sure if it makes sense to allow the user to pass in
arguments instead of hard-coding values here, but personally I think
this is ok since I have seen other packages do the same. As an example,
see
https://github.com/rksm/org-ai/blob/812b59f88851536ed3ded199fb55d303c16c7c12/org-ai-openai.el#L190-L191

* Add instructions for using auth-sources to authn

https://linear.app/mercury/issue/84

Author: tillydray

README.md

@@ -158,6 +158,14 @@ the full response of a `GetCompletions` request:
 Note that, among other things, you get probabilities for each token!
 We would love to see a PR or your own package that uses those!
 
+### 🔓 Authentication
+If you want to authenticate automatically, add your codeium api key to one of `auth-sources`. For example
+
+~/.authinfo.gpg:
+``` text
+machine codeium.com login apikey secret <insert_api_key_here>
+```
+
 ## 💾 Installation Options
 
 ### ➡️ straight.el

codeium.el

@@ -184,10 +184,12 @@
 ;; for AcceptCompletion
 (codeium-def codeium/completion_id (_api _state val) val)
 
-;; alternative getting key from file?
-;; TODO
-;; (setq codeium/metadata/api_key 'codeium-default/metadata/api_key)
-(defun codeium-get-saved-api-key ())
+(defun codeium-get-saved-api-key ()
+  "Retrieve the saved API key for codeium.com from auth-source."
+  ;; Ensure the auth-source library is loaded
+  (require 'auth-source)
+  (auth-source-pick-first-password :host "codeium.com" :user "apikey"))
+
 (codeium-def codeium/metadata/api_key (_api state)
     (if-let ((api-key (or (codeium-state-last-api-key state) (codeium-get-saved-api-key))))
         (setq codeium/metadata/api_key api-key)