Add audio-conditioned music cover client

[octo-patch]

Reason: Add audio-conditioned music cover requests to the standalone Gradio tooling.

This adds a dependency-free cover client and command-line entry point with:

• validated URL, base64, and preprocessed feature inputs
• global and China regional endpoints
• URL and hex output with MP3, WAV, and PCM settings
• generation status, service error, and audio response parsing
• focused unit coverage and local usage documentation

Checks:

• python3 -m unittest discover -s tests -v
• python3 -m py_compile gradio/music_cover.py tests/test_music_cover.py
• python3 gradio/music_cover.py --help
• git diff --check