Replace conda-build v1 recipe subprocess calls with Python API

[danyeaw]

Checklist

• I added a descriptive title
• I searched open requests and couldn't find a duplicate

What is the idea?

Replace subprocess calls with py-rattler-build Python API for building and rendering v1 recipes.

Why is this needed?

Better error handling, progress reporting, and access to build metadata.

What should happen?

1. Add py-rattler-build to dependencies
2. Create private conda_build/_rattler_integration.py module
3. Refactor run_rattler_build() in main_build.py and main_render.py to use Python API
4. Pass through CLI arguments to py-rattler-build
5. Add tests and documentation

Additional Context

Plan right now is to keep py-rattler-build APIs private (not exposed in conda_build.api)

[jsmolic]

I'll pick this issue up.