craft/base/Element::getSupportedSites() add event to modify sites for an element

[Anubarak]

We currently have a multisite setup with all in all 300 sites.
Each of these sites has many assets, currently this is a Craft 3.0 setup that needs to be updated.

In Craft 3.0 we changed the elements service in order to stop asset propagation otherwise we end up with a 100gb database just due to the amount of assets (we implemented this "fix" later and went from DB size 100gb down to 40gb).
Those assets should not be propagated and should actually be hidden from other sites, each site has it's own unique assets.

Since the required functions in Craft 4 are either private or static (as they were in later versions of Craft 3) it's much harder to achieve that goal.
It would be awesome if there was an event to modify the supported sites of an asset. I could create a pull request for this as well.