DS-7: Opaque colours

Add Opaque colours. Allow also rgba values displayed in the story, add Opaque category and add background colour for Opaque category colours.

Demo: https://uh-design-system.pages.helsinki.fi/component-library/ds-7-opaque/?path=/story/foundations-colours--colours

Jira: https://jira.it.helsinki.fi/browse/DS-7

Figma: https://www.figma.com/design/XHPvq3wuJA9eYAbU9b9NeI/University-of-Helsinki-Design-System-%E2%80%93-2024?node-id=1642-10452&node-type=frame&t=QAj4mx1JnyDaX6xR-0