Add proxy links to AP objects via FEP-fffd

[snarfed]

@silverpill recently pointed me to @ar-nelson's https://socialhub.activitypub.rocks/t/fep-fffd-proxy-objects/3172 , which defines some simple rules for using Links with optional rel-alternate/canonical in AP objects to formalize bridging relationships with their copies in other places. Exactly what we're doing here. Let's add them!

[snarfed]

FEP: https://codeberg.org/fediverse/fep/src/branch/main/fep/fffd/fep-fffd.md

Lots of interop testing discussion in the socialhub thread, specifically multi-valued and non-http url values. Seems mostly ok but probably still a bit fragile across the ecosystem.