curl backend doesn't use system certificates on MacOS

I have duckdb installed from homebrew on MacOS. I just installed the 1.5.0 update, which enabled the `curl` backend by default.
We use the `httpfs` extension for accessing S3 compatible object stores with self-signed certificates. The certificates are installed to the MacOS certificate store.

The old `httplib` backend would automatically trust those certificates, while the new `curl` backend fails with
```
IO Error:
SSL peer certificate or SSH remote key was not OK error for HTTP HEAD to 'https://***'
```

Linux is not affected by this.

It's possible to work around this problem by switching back to the `httplib` backend using `SET httpfs_client_implementation=httplib;`

Start with the httpfs extension's curl backend and compare its certificate handling with the httplib backend on macOS. Reproduce the HTTPS HEAD failure using a self-signed certificate installed in the macOS certificate store, then verify that the curl backend trusts it without regressing Linux behavior or the httpfs_client_implementation setting.

Written by the indexing model from the issue text.