Complete the Builder, and return the attributes contained in the builder.
The Builder will be cleared after this function call, and it is no longer necessary to use
Builder.clear] on it, although it is also permitted. The builder may be used again to build another set of attributes after
this function call.
The returned set of attributes is floating, and should either be passed to another gck library function which consumes this floating reference, or if you wish to keep these attributes around you should ref them with ref_sink and unref them later with unref.
a floating reference to the attributes created in the builder