Summary of Changes

This pr adds the same logic for installer plugins to be able to modify the update site url and headers as when the extension is downloaded. A use case for this pr is to add some authentication before fetching the update site url like a bearer token, the download id or similar. For extension developers it has an additional benefit to install a plugin and modify update sites for specific clients without the extra step and a modification of the update site url in the database.

Testing Instructions

• Install an older version of an extension or DPCalendar Free when you don't have one
• Go to /administrator/index.php?option=com_installer&view=update
• Click on "Check for updates"

Actual result BEFORE applying this Pull Request

Update can be found.

Expected result AFTER applying this Pull Request

Update can be found.

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed