Configurable Advanced Frontend UI Customizations

[tokinrandr]

The request is to add the possibility for countries to make customizations that allow them to add custom frontend fonctionalities, designs or even interactions to external resources

In this way, in the country config we can add custom local JS and CSS files or allowed links to external resources form given URLs. Then the core will load and activate them on frontend UI (client or login)
For example:

• Add simple UI personalizations such as interactions, styles, buttons or simple links to a custom application, etc.
• Add a tool to collect user feedbacks directly
• Add banners, pop-ups, or sections for user notifications loaded from external resources
• Add plugins or tools, such as Google Analytics, if relevant

It's up to the country what to include and ensure safe and trustworthy customizations (controlled third applications or systems are recommended)

Related issue: #9712

We have made implementations that are working well for us. You can see the changes in these PRs:

• Core: #9720
• Country Config: #787