add custom http headers when downloading packages

[waruqi]

For all http urls

package("xxx")
    set_policy("package.download.http_headers", "TEST1:foo", "TEST2:bar")

For the given url

add_urls("https://github.com/madler/zlib/archive/$(version).tar.gz", {
    http_headers = {"TEST1: foo", "TEST2: bar"}
})