Add a layer of safety around required ActivityPub Actor fields (#4703)

* fix(ap): add safe constructors, getters, and validators to ActivityPub actors to address #4701

* chore: replace nil check with the new Validate() method

* chore(test): added test to verify the values extracted from actors

* fix: return the specific error type + test for it

* fix(ap): add recovery for each AP inbox worker for a worst case scenario

* fix(ap): add additional safe accessor methods to other AP entities other than actors

* chore(tests): add tests for the new safe accessors

* fix(ap): handle empty public keys in AP actors

activitypub/inbox/workerpool.go

@@ -37,7 +37,14 @@ func worker(workerID int, queue <-chan Job) {
 	log.Debugf("Started ActivityPub worker %d", workerID)
 
 	for job := range queue {
-		handle(job.request)
+		func() {
+			defer func() {
+				if r := recover(); r != nil {
+					log.Errorf("Recovered from panic in ActivityPub worker %d: %v", workerID, r)
+				}
+			}()
+			handle(job.request)
+		}()
 
 		log.Tracef("Done with ActivityPub inbox handler using worker %d", workerID)
 	}