Convert buffer metadata getter/setter methods to Python properties for
more intuitive access. This makes the API more Pythonic by allowing
direct property assignment (buf.pts = 42) instead of method calls
(buf.set_pts(42)).
Updated properties: flags, pts, dts, duration, offset, offset_end
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9134>
e2f54cc405