AP: Support collections other than followers as recipients.

**Issue**
Currently, activities can only be addressed to individual actors or to the followers collection. Other collections, like the `apods:contacts` are not resolved.

https://github.com/assemblee-virtuelle/semapps/blob/master/src/middleware/packages/activitypub/services/activitypub/subservices/activity.js#L49-L59

**Components affected**
SemApps ActivityPub package.

**Proposal**
Support resolving arbitrary collections of actors, as long as they are in the actor's pod.

The relevant entry point is src/middleware/packages/activitypub/services/activitypub/subservices/activity.js#L49-L59; start by tracing how followers and other recipients are currently resolved. Confirm the behavior for pod-local collections such as apods:contacts; done means arbitrary collections of actors in the actor's pod are resolved as recipients.

Written by the indexing model from the issue text.