Add shared inbox support for ActivityPub delivery (#4755)

* feat(ap): add support for shared inboxes to reduce outbound load

* feat(db): refactor ap followers db into followers repository

* fix(ap): use the updated activity library to pull out the shared inbox endpoint

* chore(deps): point at updated build of owncast/activity

* fix(ap): typeless endpoints

* feat(test): update ActivityPub test to support shared inboxes

* chore(test): remove unused variable

* fix: feedback from review. Guard against SSRF/non-HTTPS/local and handle transaction errors

db/models.go

@@ -1,6 +1,6 @@
 // Code generated by sqlc. DO NOT EDIT.
 // versions:
-//   sqlc v1.15.0
+//   sqlc v1.30.0
 
 package db
 
@@ -20,6 +20,7 @@ type ApAcceptedActivity struct {
 type ApFollower struct {
 	Iri           string
 	Inbox         string
+	SharedInbox   sql.NullString
 	Name          sql.NullString
 	Username      string
 	Image         sql.NullString