Plugin API hook for Shared Channel file attachment sync (#25874)

* option for auto inviting plugin to all shared channels.

* auto-invite remotes to shared channels when flag set

* fix unit test

* immediately ping new remotes; fix unique siteurl bug

* make i18n-extract

* fix translations

* plugin hooks for file attachments

* hook for profile image sync

* fix profile image sync

* fix unit test

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/platform/services/sharedchannel/util.go

@@ -130,3 +130,21 @@ func isNotFoundError(err error) bool {
 	var errNotFound *store.ErrNotFound
 	return errors.As(err, &errNotFound)
 }
+
+func postsSliceToMap(posts []*model.Post) map[string]*model.Post {
+	m := make(map[string]*model.Post, len(posts))
+	for _, p := range posts {
+		m[p.Id] = p
+	}
+	return m
+}
+
+func reducePostsSliceInCache(posts []*model.Post, cache map[string]*model.Post) []*model.Post {
+	reduced := make([]*model.Post, 0, len(posts))
+	for _, p := range posts {
+		if _, ok := cache[p.Id]; !ok {
+			reduced = append(reduced, p)
+		}
+	}
+	return reduced
+}