Jetpack fonts: improvements

[simison]

Impacted plugin

Jetpack

What

Performance and other improvements to Jetpack Google Font module.

TODO

• [ ] Avoid too big API requests and cache objects
• [x] WP Fonts API deprecation changes in GB 14.9 once the API stabilizes in Gutenberg. PR
• [ ] After above performance issue is sorted, ensure all previously added (one, two) new fonts are re-added if they were reverted:
  • https://github.com/Automattic/jetpack/pull/28122
  • https://github.com/Automattic/jetpack/pull/28123
• [ ] Use dynamic browser agent; we now request API always as "Firefox" while Google API actually returns different response depending on browser.