Add support for passing headers

[ntindle]

Add support for passing headers to the download command in the script.

Our use case is GitHub CI pipelines downloading the binary from GitHub. If we do it more than 60 times/minute, we get rate limited.

This is the header we need for ours to work in curl:

--header 'authorization: Bearer ${{ secrets.GITHUB_TOKEN }}'

[ntindle]

Another way I could see this working is if GITHUB_TOKEN is set, send it along with curl or wget