ENH: Add default factory initialization

[tbirdso]

Leverages the InitFactory mechanism introduced in ITK commit c266651
to register VkFFT GPU-accelerated implementation classes as the default
backends for ITK FFT classes via the object factory.

With these changes in place, GPU acceleration of ITK FFT image filters is achieved by simply installing the itk-vkfftbackend Python module:

> pip install itk itk-vkfftbackend
> python
>>> import itk
>>> fft_filter = itk.ForwardFFTImageFilter.values()[-1].New()        # base FFT filter instantiated
>>> t1 = itk.VkForwardFFTImageFilter.values()[-1]                  # Prove instance is VkFFT backend with a cast
>>> t1.cast(fft_filter)
<itk.itkVkForwardFFTImageFilterPython.itkVkForwardFFTImageFilterIF3; proxy of <Swig Object of type 'itkVkForwardFFTImageFilterIF3 *' at 0x0000016275980270> >

This PR relies on ITK features that will be available in the ITK 5.3rc04 prerelease, may need to wait for that release in order for CI to pass. I have verified tests pass on my local Windows system when built against the ITK main branch.

[thewtex]

💯 🥇

[tbirdso]

Rebased on master.

[tbirdso]

Linter failure tracked in InsightSoftwareConsortium/ITKSplitComponents#52 and does not originate from these changes.