feat(gallery): add thumbnail - FRONT-4565

[emeryro]

• add possibility to have a thumbnail, different from the full picture
• this thumbnail is optional; if not provided, the main image will be used as it was the case before

[github-actions]

🚀 Deployed on https://ecl-preview-3553--europa-component-library.netlify.app

[planctus]

mmh..i understand that this new feature might actually come from the request to fix a "wrong" behavior in the overlay, but this way we are not really doing much to improve the situation we had regarding the amount and the heaviness of the images loaded, actually we are doing worst, since now we will have two picture objects per items, both images we will be downloaded by the browser.
To me we should do more or less what we have been doing in BCL, where we set the "src" attribute via js when the overlay opens, so that it will only be downloaded when it is requested in the overlay.

[planctus]

• We cannot use the same picture element for the thumbnail and the overlay, if sources are provided nothing is ensuring that the "thumbnail" image will be shown in the gallery list.

• The previous implementation seemed ok, we probably don't need to have a picture element for the thumbnail itself, a simple img could be fine for that, but in any case the problem was that the second picture element had a src attribute defined, and with that the image would have been downloaded at page load.

So the idea was just to set an attribute in the second (hidden) picture element instead of the src, for then replacing this via js when needed.

• there is a problematic css at line 96, with multiple sources defined each src would take some space and the rendered image would not really "cover" the available space, this currently happens both in the list and in the overlay
  

So, my proposal is:

• make the thumbnail a simple img or keeping it as a picture, but separated from the overlay one

• having a picture object for the overlay image hidden at page load

• rendering the img in the overlay picture element without the src attribute, like with a data-src attribute

• replacing the data-src attribute with src when the item is clicked

• introducing some examples with multiple sources, we have none at the moment and this is probably the reason why certain issues cannot be identified in the current demo

[planctus]

Minor things, i think we are ok with this, we should only ensure that any "attribute" they could pass before is still going to work, if none was available before, then we are safe..