[ImageField] Support constraint validation

[Seb33300]

Fixes #5227 and #4088

Currently, it is not possible to properly validate images uploaded with the ImageField using Symfony constraints.
This is because the constraints option is applied to both the parent field (returning a string) and the underlying FileType field (returning an UploadedFile) resulting in unexpected validation errors.

This PR adds a new setFileConstraints method to the ImageField to apply constraints to the FileType field only.
By default, I also applied the Image constraint on the ImageField.

Usage example:

use Symfony\Component\Validator\Constraints\Image;

// Validate that file is an image by default
ImageField::new('logo', 'Logo'),

// Set custom validation constraints
ImageField::new('logo', 'Logo')->setFileConstraints(new Image(maxSize: '100k')),

[psihius]

Great PR.
This also needs to be reflected in the documentation on the SF site, who's able to update the docs for that? :)

[Seb33300]

@psihius good point.

Just pushed a commit to update the doc.

[javiereguiluz]

Indeed, this is a very nice contribution. Thanks a lot Sébastien!