Added V15 Colab notebook, FastAPI Serving, model downloads .py script, and Dockerfile (working in MAC M3), readme updates

[rafipatel]

Description:
This PR introduces a streamlined setup and deployment workflow for MuseTalk V15, including:

• V15 Colab notebook: For fast cloud experimentation and demo.
• FastAPI serving: Enables RESTful inference via a Python API (usable in both local and production setups).
• Automated model downloads (download_models.py): Robust pure-Python script to fetch all required weights from Hugging Face, Google Drive, and direct URLs, with auto package installation.
• Dockerfile: Provides working containerization, verified on Mac M3 hardware.

Why was this needed?
The previous download_weights.sh script frequently failed due to issues with huggingface-cli, including authentication errors and version mismatches across environments.
This PR replaces it with a portable Python script that is more reliable, does not require manual CLI tooling, and works "out of the box" for both local development and Docker-based deployments.