pipewire-[backend|plugin]: Add timestamps to buffers

In Pipewire and Gstreamer terminology Weston is a "live" source (as we
do not explicitly set PW_KEY_STREAM_IS_LIVE to false).
Such sources, be it compositors, cameras or microphones, usually set
the current system time as timestamps on buffers in order to make life
easier for consumers. Thus let's do so as well.

This notably helps when recording using `gstpipewiresrc` with the
`keepalive-time` property set.

Signed-off-by: Robert Mader <robert.mader@collabora.com>