Add Tribe__Cache::has method

[lucatume]

🎫 Ticket

Innovation Day work, Cookbook

🗒️ Description

For back-compatibility purposes, I've not modified existing methods like offsetGet. This add a $found flag, set by reference, to the get method similarly to the one set by the wp_cache_get function. To just check on a value, I've added the has method.

✔️ Checklist

• [ ] Changelog entry in the readme.txt file.
• [ ] Code is covered by NEW wpunit or integration tests.
• [ ] Code is covered by EXISTING wpunit or integration tests.
• [ ] Are all the required tests passing?
• [ ] Automated code review comments are addressed.
• [ ] Have you added Artifacts?
• [ ] Check the base branch for your PR.
• [ ] Add your PR to the project board for the release.