ActivityPub Client-to-Server experiments

[lanodan]

Btw I tagged the issues and merge requests related to AP C2S in pleroma’s gitlab:

• Issues: https://git.pleroma.social/pleroma/pleroma/-/issues?label_name=AP+C2S
• Merge Requests: https://git.pleroma.social/pleroma/pleroma/-/merge_requests?label_name[]=AP+C2S