element: Add sanity check with a critical warning if a pad is requested for a pad...

element: Add sanity check with a critical warning if a pad is requested for a pad template that is not installed on the element class

Various elements are assuming that the pointer matches a pad template
they know about, and also randomly created pad templates might be
missing some important information that is necessary to create a valid
pad.

For example, creating a new pad template for audiomixer's sinkpad
without providing the correct GType would cause audiomixer to create a
GstAggregatorPad. That will then later fail spectacularly because it
assumes that it got a GstAudioAggregatorPad.

Passing a pad template that does not belong to the element class in here
will easily lead to undefined behaviour.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2410>