va: Install library and generate its GI
Copied from https://gitlab.freedesktop.org//gstreamer/gst-plugins-bad/-/merge_requests/2259
Currently libgstva is only exposed internally to gst-plugins-bad. No headers are installed, no documentation generated, neither gobject-introspection files.
This patch turn on all if we decide to offer the library to users of gst-plugins-bad.
Documentation generation is dropped because the plugin is not currently build by CI.
As in the case of d3d11, no API stability promises are made.