overrides: add a set_caps() method to the Pad override

The C API provides the gst_pad_set_caps() helper which makes it easier
to set caps on pads (see gst/gstcompat.h in gstreamer core).

Add such handy helper to the python bindings too.

The implementation follows as close as possible the one in gstcompat.h
with two changes:
  1. the type check on the pad has been removed because self is
     guaranteed to be a Gst.Pad in python.
  2. the null check on the caps has been extended to be a type check.

Fixes gstreamer/gst-python#19
8 jobs for add-Gst.Pad.set_caps in 159 minutes and 45 seconds (queued for 91 minutes and 9 seconds)
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gst indent

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manifest

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build fedora x86_64

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  Test
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build android arm64 api28

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build cerbero cross win32

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build cerbero cross win64

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passed #76598
build cerbero fedora x86_64

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