[Feature request] Add "boxed" variant to the Testimonial

[seangolob]

Hi PB Team!

I have noticed a few pages that have testimonials that have a boxed wrapper around them:

Single Testimonial:

https://github.com/features/actions
https://github.com/features/issues

Two Testimonials:

https://github.com/features/codespaces
https://github.com/features/code-search

Can we consider adding a "boxed" variant to the Testimonial component to support these use cases?

The Single Testimonial page examples have a somewhat transparent background with a blur which should probably exist at the implementation level. However, it would be nice to have the spacing defined in the PB library to have future testimonials share the same spacing.

CC @simmonsjenna CC @jesussandreas

Thank you for your help!