Need API to set the default Cert Store location at runtime

For some time, we at GStreamer have been carrying a patch to glib-networking that allowed Android and iOS apps to set the ca certificate store location at runtime with an environment variable. It was not upstreamed because it was a hack.

This patch was needed by Android and iOS apps because they do not know where the certificate store is actually located till they are run (the cert store is bundled with the app, and the app install path is unknown at compile time).

A few months ago, the ability to set the system store location at compile-time was removed, and this broke our patch. There is no system trust on Android/iOS, so we need a way to set the default certificate store to use at runtime.

Apps that use gnutls or openssl generally use APIs provided by both to set the store locations (f.ex. SSL_CTX_set_default_verify_paths()), so we need to expose that in glib-networking too.