gtlsbackend: add support for setting the default TLS database
There are many cases where a default TLS database is not able to be defined within the constraints of a system. For example glib-networking (or glib-openssl) cannot retrieve the default certificate store on iOS or Android and need to be initialized from a cert file of certificates bundled with the application.
Previously GStreamer was relying on a custom patch to glib-networking to populate the default database from the file pointed to by the CA_CERTIFICATES environment variable however the mechanism that enabled this was recently remove from glib-networking.
Adding a more generic g_tls_backend_set_default_database() API allows application developers to override the default database using their own certificates as well as allowing equivalent functionality on Android/iOS (or others) as on the default database handling Linux.