Support configuring a list of site URLs

Nobody has claimed this yet.

see https://matomo.org/faq/how-to/how-do-i-add-a-referral-exclusion-in-matomo/

In Matomo for WordPress this feature is currently not available.

Not sure how we could let a user configure additional URLs best? As it's rare maybe it would be enough to let users define a constant in wp-config.php and we add the configured URLs to the list of site urls. Not sure though if we can already hook into this and dynamically add these sites. However, we could potentially read the constant defined in wp-config.php and then configure this as part of the site/sync scheduled task if needed. However, this would mean that it would take a while for a constant change to be active which may not be what users expect and they'd think the feature is not working.

Possibly we can hook into the `Site.setSites` event and add more sites dynamically, maybe.

The goal of this issue is to make the referral exclusion work without having this manage measurable page.

Start with the linked referral-exclusion FAQ and investigate whether the Matomo `Site.setSites` event can add configured URLs dynamically. Compare that with reading a constant from `wp-config.php` and determine how configuration changes would be applied. Done means referral exclusion works without requiring a manageable site.

Written by the indexing model from the issue text.