daemon: Add a synchronous version of goa_provider_get_all
daemon: Add a synchronous version of goa_provider_get_all

This will be useful to poke at all available providers when goa-daemon
starts. Since we are doing this in the early stages of start-up, it
would be nice to be able to get the list of providers synchronously.

This function is not suitable for wider use because we can't push a
new thread-default GMainContext due to limitations (or bugs) inside
telepathy-glib.

https://bugzilla.gnome.org/show_bug.cgi?id=766733

ritzk/gnome-online-accounts,ritzk/gnome-online-accounts,ritzk/gnome-online-accounts