Enhancement: Backup - update setup page pricing UI to show loading while price loads

### Impacted plugin

Backup

### What

Update UI to show a loading indicator while price loads.

### How

When the request to fetch the price takes a while, the price says 0 and that is misleading for the user.

https://github.com/Automattic/jetpack/assets/11799079/969899a2-8264-44cb-b3d0-3c70c1ba12ca

Here's what Social does (notice the loading indicator after refreshing the page):

https://github.com/Automattic/jetpack/assets/11799079/e91a8114-70c3-450a-98b8-a8708e7c5b12

Start by tracing the Backup setup page's price-fetch request and compare its behavior with the loading indicator shown in Social. The work is done when the page shows a loading state while the price request is pending instead of displaying 0 as the price.

Written by the indexing model from the issue text.