tracing: add hooks for gst_pad_chain() / gst_pad_chain_list()

This allows tracing buffers when they arrive in a pad instead of just
when they are pushed out of a pad.

Part-of: <gstreamer/gstreamer!2682>
27 jobs for !2682 with tracer-pad-chain in 31 minutes and 19 seconds (queued for 3 seconds)
merge request