[@rancher/components] Card.vue: Toggle box-shadow

[rak-phillip]

The default styles for Card.vue contain a box-shadow. There are multiple instances where the box-shadow is undesirable and it would be nice if the box-shadow could be disabled for the card component.

I think the best course of action would be to remove the box-shadow entirely and allow the consumer to define box-shadows as needed, but I can see how this would be a regression in existing styles so it's probably best to keep box-shadow enabled by default and allow for disabling it.

🔳 Acceptance Criteria

• [ ] When the box-shadow is disabled, the card component should have no visible box-shadow.
• [ ] When the box-shadow is enabled, the card component should have the default box-shadow as defined in the current styles.
• [ ] The box-shadow toggle should be easily accessible to the consumer, such as a prop or a CSS class.
• [ ] Enable the box-shadow by default.
• [ ] The toggle should work consistently across different browsers and devices.
• [ ] The code changes should be backwards compatible and not break any existing functionality.
• [ ] Update storybook to include information about the new toggle feature and how to use it.
• [ ] Add unit tests to ensure the toggle works as expected.
• [ ] The toggle should not affect any other styles or functionality of the card component.

[gaktive]

Depends on what's decided on rancher/dashboard#8859 for the overall approach to cards.