giosrc: Add support for growing source files

giosrc: Add support for growing source files

Add a way for applications to specify that the underlying file is
growing which implies that the source won't EOS when reaching the end
of the file but instead start monitoring it and start reading it again
whenever a change is detected.

Also add a validate test to check the behavior

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/937>