Make sure repository uses Opencat PHP library

[ferishili]

This PR fixes #38,

Description

After reviewing the usage of the old cURL API call in the tool_opencast plugin, I identified only one instance where the get method is being used.

Solution

This PR contains using the opencastapi property within the api class of the tool_opencast plugin while ensuring that the required behavior remains unchanged.

Testing Instructions

1. Apply this PR to your setup.
2. Ensure that the plugin is correctly configured.
3. Use the plugin alongside the filter_opencast plugin to access the video selection interface from the Opencast repository.
4. Verify that the displayed list of videos is correct and includes all videos from the selected series.