Improve activity pub actors implementation

server/helpers/activitypub.ts

server/helpers/custom-validators/activitypub/activity.ts

server/helpers/custom-validators/activitypub/misc.ts

server/lib/activitypub/actor.ts

server/lib/activitypub/process/process-accept.ts

server/lib/activitypub/process/process-reject.ts

server/lib/activitypub/process/process-undo.ts

server/lib/activitypub/process/process.ts

shared/models/activitypub/activity.ts