Allow users to override index `cache-control` headers (#14620)
## Summary
You can now override the cache control headers for the Simple API, file
downloads, or both:
```toml
[[tool.uv.index]]
name = "example"
url = "https://example.com/simple"
cache-control = { api = "max-age=600", files = "max-age=365000000, immutable" }
```
Closes https://github.com/astral-sh/uv/issues/10444.
