Add API route for high fives

# Requirements

- [ ] Authenticated API route is created
- [ ] Authentication does not require to be logged in, there is some other auth mechanism that an admin would setup
- [ ] Authentication approach uses the documented "correct" approach, or uses an approach that we observe in other projects to be best practice
- [ ] We document the source of why we think that is the "correct" approach (i.e. documentation on Moogle.org) or best practice (e.g. forum posts, other open source projects) where we see a thoughtful discussion of making this choice
- [ ] The route causes a high five to happen
- [ ] REST route documentation is added to readme

# Discuss

- [ ] Here is one example implementation https://github.com/fulldecent/moodle-enrol_course_tokens/pull/55

Here is an example of defining an API integration but not using a hardcoded API key:

- https://github.com/mdjnelson/moodle-mod_customcert/blob/d7fcb8aef43a3b03e44173a460a09f3573b365e9/db/services.php#L30

# Follow on work

- [ ] Update [moodle-enrol_course_tokens](https://github.com/fulldecent/moodle-enrol_course_tokens)
- [ ] Use this approach
- [ ] Document this feature
- [ ] Link back to this plugin as citation demonstrating that is the best practice (and/or official documentation if it is best practice)

Start by reviewing the README and the linked example implementations, including db/services.php in moodle-mod_customcert and the moodle-enrol_course_tokens pull request. Determine and document an appropriate non-login authentication approach, create the authenticated route that triggers a high five, and add REST route documentation to the README.

Written by the indexing model from the issue text.