Add requirements.txt for dependencies

Install Python dependencies required for the Gradio app:
- gradio: Web interface framework
- pandas: Data manipulation and analysis
- numpy: Numerical computing
- scikit-learn: Machine learning library
- joblib: Model serialization

This will resolve the runtime error and enable the churn prediction demo.

requirements.txt

@@ -0,0 +1,5 @@
+gradio>=4.0.0
+pandas>=1.5.0
+numpy>=1.21.0
+scikit-learn>=1.1.0
+joblib>=1.2.0