Add dev_config.yaml for local proxy development

Provides a minimal config for running the proxy server locally:
- fake-openai: Uses a mock endpoint for testing without real API keys
- gpt-4o: Uses OPENAI_API_KEY from environment when available
- master_key: sk-1234 for local development

Co-authored-by: Ishaan Jaff <ishaan-jaff@users.noreply.github.com>

dev_config.yaml

@@ -0,0 +1,13 @@
+model_list:
+  - model_name: fake-openai
+    litellm_params:
+      model: openai/fake
+      api_key: fake-key
+      api_base: https://exampleopenaiendpoint-production.up.railway.app/v1
+  - model_name: gpt-4o
+    litellm_params:
+      model: openai/gpt-4o
+      api_key: os.environ/OPENAI_API_KEY
+
+general_settings:
+  master_key: sk-1234