Widgets: Display WordPress Posts widget performance issues

The widget makes a network request for each page load. That request should be cached. Further, we should limit the response to the fields that we need.

See pau2Xa-1rU-p2 for context

#### Steps to reproduce the issue

See the post for more information.

#### What I expected

- Network calls to be cached
- Fields to be limited to what is necessary

#### What happened instead

- Network calls not cached
- All default fields returned

**Screenshots**
If applicable, add screenshots to help explain your problem.

Start with the WordPress Posts widget and the linked context reference, then trace the request made during page loads. The work is done when that request is cached and its response is limited to the fields the widget needs.

Written by the indexing model from the issue text.