Gst.init() has to be called before GstPbutils is imported

This makes sure that we do not try to use GstPbutils before Gst is init
and in case GstPbutils is imported while Gst is not imported, use the
`GstPbutils.pb_utils_init()` function to have the oportunity to
initialize the overrides.

Not that we also introduce a `GstPbutils.init()` variant because
`GstPbutils.pb_utils_init()` is an ugly name.
8 jobs for master in 16 minutes and 51 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #94283
gst indent

00:00:16

passed #94282
manifest

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  Build
passed #94284
build fedora x86_64

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  Test
passed #94285
check fedora

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  Full Builds
passed #94286
build android arm64 api28

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passed #94288
build cerbero cross win32

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passed #94289
build cerbero cross win64

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passed #94287
build cerbero fedora x86_64

00:08:38