Allow file variant with different extension

[maxime-rainville]

As a developer, I want to be able to create variant of files with different extensions.

Acceptance criteria

• File URL of variants can still be resolved back to their parent file regardless of whether they have a different extension. (View FileIDParser)
• You can still have file with the same name and a different extension and avoid collision.
• Normal file operation like session grant, deletion, move, publication or archival apply to variant with different extensions as well.
• There's an API for creating variant with alternative extensions.
• Variants with different extensions can be created for regular files as well as image.
• All Image manipulation functions work for variants with alternative extensions if they map to an image format.
• All Image manipulation functions can be chained when applied to an image variant with a different extensions

Follow up task

We've decided te spin off the follow AC as it's own card:

• Variants of Variants are handled in a sensible manner and avoid needless duplication.

This is meant to illustrate what "Variant of Variant" means. Note that those image conversion methods won't exist right away.

• $JpegImage.Png().Jpeg() doesn't throw an error or create a duplicate of the original image.
• $JpegImage.Png().WebP() doesn't throw an error, doesn't use an intermediary step and creates the same file has calling $JpegImage.WebP().
• $JpegImage.FitMax(100,100).WebP() results in exactly the same file as $JpegImage.WebP().FitMax(100,100)

Notes

• There's a proof of concept that implements all of these concepts.
  • There's a low level manipulate API for creating variant with alternative extensions (something similar to manipulateImage)
• What happens when you upload image1.jpg, which has a webp variant, and then you upload a separate image1.webp? When you try to download https://example.com/assets/image1.webp which of the files do you get? Or does the existence of image1.jpg stop you from uploading image1.webp?
  • The URL for the variant webp is not just assets/image1.webp so there should be no conflict, but this should be validated.

Excluded

• Generic ToImage API that would attempt to create Thumbnails for any file type.
• Higher level Converter service API ... that will come later (see API/Service for registering "converters" that will create file variant under a different extensions #489)

PRs

• ENH Refactor FileIDHelper classes to reduce code duplication #587
• NEW Allow file variants with different extensions #585
• DOC Document making variants with different file extensions developer-docs#441