urisourcebin: Refactor ChildSrcPadInfo and OutputSlot usage

urisourcebin: Refactor ChildSrcPadInfo and OutputSlot usage

Make an explicit topology/tree of structures:
* ChildSrcPadInfo is created for each source element source pad
* ChildSrcPadInfo contains the chain of optional elements specific to that
  pad (ex: typefind)
* A ChildSrcPadInfo links to one or more OutputSlot, which contain what is
  specific to the output (i.e. optional buffering and ghostpad)
* No longer use GObject {set|get}_data() functions to store those structures and
  instead make them explicit
* Pass those structures around explicitely in each function/callback

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2784>