Add a handle parameter to pass to curl::curl_download function

[Sdurier]

basically i add a handle parameter to function font.add.google and download_font_file, with default value curl::new_handle() which is the default value in curl::curl_download.

[yixuan]

Thanks! Since I just updated sysfonts recently, it may take one or two weeks to submit the new version per CRAN's policy. I'll document your contribution in the NEWS file later.