Plugin API for Shared Channels: support auto invite (#25834)

* option for auto inviting plugin to all shared channels.

* auto-invite remotes to shared channels when flag set

* fix unit test

---------

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

server/platform/services/sharedchannel/util.go

@@ -105,6 +105,10 @@ func mungEmail(remotename string, maxLen int) string {
 }
 
 func isConflictError(err error) (string, bool) {
+	if err == nil {
+		return "", false
+	}
+
 	var errConflict *store.ErrConflict
 	if errors.As(err, &errConflict) {
 		return strings.ToLower(errConflict.Resource), true
@@ -117,3 +121,12 @@ func isConflictError(err error) (string, bool) {
 	}
 	return "", false
 }
+
+func isNotFoundError(err error) bool {
+	if err == nil {
+		return false
+	}
+
+	var errNotFound *store.ErrNotFound
+	return errors.As(err, &errNotFound)
+}