Introduce gst_element_request_pad_simple

Introduce gst_element_request_pad_simple

The name `gst_element_get_request_pad()` is confusing to people
learning GStreamer. `gst_element_request_pad_simple()` aims at
providing the exact same functionality, while making it more
explicit it is a simplified `gst_element_request_pad()`.

`gst_element_request_pad_simple()` is consistent with other
functions such as `gst_element_seek_simple`.

This commit deprecates `gst_element_get_request_pad()` so that a
compilation warning is emitted when used and incite developers
to use the more explicit `gst_element_request_pad_simple()`.

See also https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/743#note_886586

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/802>