Support for changed owners

We currently support listing changes of authors, as stated inside the `Cargo.toml`. We would also like to be able to detect change of _owners_ ‒ the people who are allowed to publish the crate. If someone does a takeover, we should know about it, but they might not always update the authors field.

On the other hand, currently there doesn't seem to be a good API to find it out, on the client side. So the way to get the information needs to be researched.

Start by reviewing the existing author-change handling described in Cargo.toml and researching how a client can detect changes to the people allowed to publish a crate. Determine whether a usable API exists and document the approach needed to identify takeovers when the authors field is unchanged.

Written by the indexing model from the issue text.