feat(select): Grouping capabilities for simple and multiple select - FRONT-3542

[planctus]

• this is only adding the support for optgroups in the default and multiple select
• in the mocks there are now styles for the default select that were not foreseen and cannot be reproduced simply with some css, these styles have not been "reproduced".
• It is pretty limited as a functionality, we do support groups in the multiselect but as simple strings, no functionality is associated to it (like selecting all the options in the group)

[github-actions]

🚀 Deployed on https://ecl-preview-2712--europa-component-library.netlify.app

[emeryro]

The hover area of item in groups is quite strange

I guess that it is a mistake, as the click area goes on the full width

[emeryro]

Also the cursor does not seem consitent. For instance title have a hand cursor while they are not clickable, and hover area has a default cursor