[Feature] add EDU images as an external source

## Problem
The goal is to add a good number of external sources to give users a variety of results from different sources.

## Description
[EDU images](https://images.all4ed.org/) ia a free library of photos celebrating students—and the educators who teach them—in seven schools across the United States, licensed under [Creative Commons Attribution-NonCommercial 4.0 International](https://creativecommons.org/licenses/by-nc/4.0/)..Gotten from the additional sources list of Openverse available in the file [get-additional-sources.ts](https://github.com/WordPress/openverse-frontend/blob/main/src/utils/get-additional-sources.ts#L61) mentioned in https://github.com/creativecommons/search/issues/48

Adding EDU images as an external source to CC search will be a good addition and will benefit our users.

## Alternatives

## Additional context

## Implementation

- [x] I would be interested in implementing this feature.

Start with src/utils/get-additional-sources.ts in the Openverse frontend, especially the existing additional-source entries and the EDU Images listing linked in the issue. Add EDU Images as an external source with its attribution and licensing details, then verify that the source appears in the CC Search results and follows the existing source format.

Written by the indexing model from the issue text.