Export images uploaded via CKEditor

Attached patch provides this feature, based on already-existing examples.

$this->privateTempStore->get('export.assets')

What is going to flush out things stored in this storage after the export is done? I see that whatever is stored there gets overridden every time. By the way, due to this user-specific but shared storage, isn't there a slight chance for unexpected behaviors when the same user executes multiple exports at the same time?

@mxr576: While your comment is asking a proper question, it does not belong to this particular patch/issue. The race condition you mentioned is also present without this patch, as this user-specific but shared storage is also used elsewhere already during the export process. Because of that, I think a different issue should handle that problem – if at all, because I also think that this is quite a corner case: I just cannot really imagine myself executing another export while the first one also started by myself is still running. IOW, this is theoretically possible for sure, but practically? I'm not that sure.

roger :) thanks for the detailed answer

The patch in #2 didn't work for me. I received an error when trying to export. I changed line 45 from

public function mapBaseFieldsValues(array $values, FieldableEntityInterface $entity): array {

to

public function mapBaseFieldsValues(array $values): array {

and that worked for me.