Support DeviceCMYK images ColorSpace

[Lucas-C]

Currently fpdf2 converts images in CMYK into simple DeviceRGB images:
https://github.com/PyFPDF/fpdf2/blob/2.6.1/fpdf/image_parsing.py#L74

However the PDF format allows to embed images defined in CMYK ColorSpace directly.

Implementing this would also be a first step towards #573

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By implementing this feature you, as a benevolent FLOSS developper, will provide access to the large community of fpdf2 users to a useful functionality.
As a contributor you will get review feedbacks from the maintainer & other contributors, and learn about the lifecycle & structure of a Python library on the way.
You will also be added to the contributors list & map.

This issue can count as part of hacktoberfest