Opaque context structure for the plugin installation. Use the provided
API to set details on it.
The prototype of the callback function that will be called once the
external plugin installer program has returned. You only need to provide
a callback function if you are using the asynchronous interface.
whether the installation of the requested plugins succeeded or not
the user data passed to gst_install_plugins_async()
Result codes returned by gst_install_plugins_async() and
gst_install_plugins_sync(), and also the result code passed to the
#GstInstallPluginsResultFunc specified with gst_install_plugin_async().
These codes indicate success or failure of starting an external installer
program and to what extent the requested plugins could be installed.