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.

Fixes glib-networking#35