Added webservice-functions to mod_publication

[melemalex]

The publication can only be interacted with using a browser. This PR solves this problem by providing moodle webservice endpoints. Currently only student-relevant endpoints are available as this is our goal. However, the webservices can be extended easily.

To provide webservice functions for the mod_publication plugin the following additions were made:

• Added db/services.php which includes the description of the 2 endpoints
• Added tests/externallib_test.php which includes unit tests for the endpoints
• Added externallib.php which includes the actual endpoints.

Following endpoints were added:

• mod_publication_get_publications_by_courses
• mod_publication_get_publication

The collection of PRs posted by us, providing webservice functions, is supposed to serve the use-case of providing a student API for TUWEL (TU Wien Moodle Platform). It is with that purpose in mind, that we chose the plugins and the functionality to provide via web service. The following PRs are related to this effort:

• Checkmark
• Grouptool
• Offline-Quiz
• Organizer
• Publication

[goggo24]

Student API on hold - implementation of webservices in generally under discussion.