[Feature] Optional webservice to restrict access to released data

[chriso123]

To make the access to the public metadata more restrictive, a webservice should be defined that can be used with a token (moodle core functionality)

Current state:

When the public access is enabled, there are no restrictions when calling the public_metadata.php file.

Goal:

Instead of opening the system to everyone, a token authentication should be used so only privileged people/systems can access the data. (Example: only the repository should have access to the oer plugin, the released files then are handled by the repository to make them public)

The configuration of the service than can be made through Moodle default settings (Protocoll, users with token etc.)

Todos:

• Add a setting to activate the webservice for OER (is this necessary? the service itself is handled by moodle, and is protected by moodle token authentication anyway)
• Add the service definition and classes with correct return values