FEP-0837: Federated Marketplace

Updating the proposal: #318 - FEP-0837: Support donations - fediverse/fep - Codeberg.org

Following the discussion in another topic, arbitrary quantities has been allowed to support donations. I also added a sequence diagram (can be previewed at feps/0837/fep-0837.md at main - silverpill/feps - Codeberg.org).