[Feature] Add 'The Greats' 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
[The Greats](https://www.thegreats.co/thegreats/public/artworks) is a free site to discover awesome themed art illustrations. it is also 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 The Greatss as an external source to CC search will be a good addition and will benefit our users by giving them more search options.

## Alternatives

## Additional context

## Implementation

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