update thumbnail, allow user to choose output format

[kouz75]

Add output format support to thumbnail URLs

• Add optional format parameter to ThumbnailHelper, ImageExtension, and the thumbnail() Twig function
• Encode the output format as a filename suffix in the URL (e.g. image.jpg.avif) instead of in the param string
• Parse the format suffix in ImageController to set Glide's fm parameter and resolve the correct source file
• Backward compatible: URLs without a format suffix continue to work unchanged

Update thumbnail file path in Twig and filesystem to match image URL structure, allowing Apache/Nginx to serve them directly without any PHP call

[Vondry]

As a general question - is there a command that would "warm up" thumbnail cache so that even first user can benefit from fast loading image?

[kouz75]

There is no command for that yet.
One way to fix this is to add a new command that parses all media files (or the content) and generates their thumbnails. The thumbnail settings can be defined in theme.yaml or passed as parameters to the command.

This command may generate a lot of thumbnails that will never be displayed on the website.

[bobvandevijver]

I will need more time to definitively check this, but here's already my first findings. I also believe that are changes in this PR that are not necessarily needed to fix the main issue (which is fixing the webserver direct file serving).

[bobvandevijver]

Why has the usage of this setting been removed?

[bobvandevijver]

Direct usage of the filename will cause a regression into a security incident (#3661) we had in the past, namely a path traversal. Paths can only be used after a canonicalize call.