Update Python version to 3.12.4 and add .env.Example file

Author: martinibach

openai_greet/.env.Example

@@ -0,0 +1,8 @@
+OPENAI_API_KEY=<your-openai-api-key>
+
+# Restack Cloud (Optional)
+
+# RESTACK_ENGINE_ID=<your-engine-id>
+# RESTACK_ENGINE_API_KEY=<your-engine-api-key>
+# RESTACK_ENGINE_ADDRESS=<your-engine-address>
+# RESTACK_CLOUD_TOKEN=<your-cloud-token>

openai_greet/README.md

@@ -31,7 +31,7 @@
 4. Install dependencies using Poetry:
 
    ```bash
-   poetry env use 3.12
+   poetry env use 3.12.4
    ```
 
    ```bash