How to retrieve "user@server.tld" handle from actor's URL

[silverpill]

FEP-2c59 introduces webfinger, a new property of an actor object: https://codeberg.org/fediverse/fep/src/branch/main/fep/2c59/fep-2c59.md

Discussion: FEP-2c59: Discovery of a Webfinger address from an ActivityPub actor