Social app & ActivityPub integration with other supported services, such as Mastodon

Hi, I would like to use ActivityPub to communicate Nextcloud's Activity to other ActivityPub-supported services such as Mastodon or Hubzilla. afaik, Integration would mean those services could also send their notifications and comments to my Nextcloud instance. Here is a useful article on setting up a basic ActivityPub server from a non-Mastodon service.

Enter ActivityPub! ActivityPub is a decentralized social networking protocol based on the ActivityStreams 2.0 data format. ActivityPub is an official W3C recommended standard published by the W3C Social Web Working Group. It provides a client to server API for creating, updating and deleting content, as well as a federated server to server API for delivering notifications and subscribing to content.

ActivityPub integration could allow Nextcloud Activity app to communicate with other federated chat / notification services, including:

Mastodon & GNU Social
Hubzilla
Friendica
Pleroma
Socialhome

Start by reading the linked ActivityPub server article and reviewing the Activity app repository entry points; the issue names no files or tests. Done would require an agreed scope and working communication between Nextcloud Activity and the listed federated services.