Refactor KeyCDN thumb handling

[lukasbestle]

Description

Summary of changes

• Refactor CDN plugin to use a separate KeyCdn class for image processing params
• Fixed use of image processing params according to KeyCDN docs
• Hook into Darkroom class for normalization
• Add support for focus point cropping and other Kirby thumb options
• Crop from the center by default (instead of the smart algorithm that tries to pick a focus point that may or may not match what we want)
• Remove cdn() helper that was only used internally in the CDN plugin

Reasoning

• Cleaned up code that is easier to keep the overview on
• Increase reliability of image processing
• Increase consistency with our local GD/ImageMagick setups
• Allow us to use the full spectrum of Kirby thumb options in the future (we could add additional ones to the KeyCdn class if needed, I've started with the most common ones we already support with ImageMagick)

Additional context

Relevant docs: https://www.keycdn.com/support/image-processing

[lukasbestle]

About the method order: I agree it would be good to use a consistent pattern in the whole team. What I like about having public methods first is that it allows to see at a glance what I can do with a class. Whereas finding a specific method is easy with Cmd-R in VS Code and similar tools in other IDEs.