secrets: use tcurl in proxy provider
We switch from http-parser to libcurl for an http client. This gaves us many
features for free such as tls and http basic authentication support instead
of implementing it on our own.
Resolves:
https://pagure.io/SSSD/sssd/issue/3192
Reviewed-by: Simo Sorce <simo@redhat.com>
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>