feat: add ability to specify a custom message for validation rules

[azjezz]

Description

When adding rule validation to a DTO, i would like to be able to specify a custom message.

Example:

use Tempest\Validation\Rules;

final class Book {
    #[Rules\HasLength(
      min: 5,
      minMessage: 'book title is too short.',
      max: 50,
      maxMessage: 'book title is too long!',
    )]
    public string $title;

    #[Rules\IsNotEmptyString(message: 'book description should not be empty!')]
    public string $description;

    #[Rules\HasDateTimeFormat('Y-m-d', 'book publish date is incorrectly formatted!')]
    public ?DateTime $publishedAt = null;
}

It would also be nice to be able to specify translation keys instead, allowing users to translate validation error messages depending on the request locale.

use Tempest\Validation\Rules;

final class Book {
    #[Rules\HasLength(
      min: 5,
      minMessage: 'book.error.title.short',
      max: 50,
      maxMessage: 'book.error.title.long',
    )]
    public string $title;

    #[Rules\IsNotEmptyString(message: 'book.error.description.empty')]
    public string $description;

    #[Rules\HasDateTimeFormat('Y-m-d', 'book.error.published_at.format')]
    public ?DateTime $publishedAt = null;
}

Benefits

More customization, and better / more specific error messages to end users.

[innocenzi]

With MessageFormat 2, there's no need to have a dedicated message for every possible variable of rule failures—it's all in the same translation unit!

#1618 will allow for specifying the translation key for fields, but it will be as a separate attribute, and we won't support full error messages directly (at least for now).