Add rate limiting for attachment extraction

Merged the MR, will be tested in HEAD.

Merged a second MR adding a warning message to the index “View” tab if some files are still pending extraction (for already indexed items).

Modified the warning message to only contain the total number of queued files since the other number could be incorrect due to #3601364: Queued files in key-value store are not properly deleted.

Released as part of version 1.12.0.

Note that in versions of the Search API Attachments module older than 10.0.3 (without the fix for #3372383: Allow extractor plugin to throw DelayedRequeueException) it was not possible to add a delay when failing extraction of a file. Therefore, when the rate limit is reached the file extraction might be retried immediately. In the worst case scenario, a file extraction might be retried five times almost instantly, always running into the same rate limit, at which point the Search API Attachments module would give up and not try extracting that file anymore.

If possible, it is therefore recommended to update to version 10.0.3 or higher of the Search API Attachments module.

This problem is mitigated by the fact that the same problem would already happen when running into the rate limit of the SearchStax Document Extractor endpoint. Therefore, there would be no additional problem, this new functionality will just not be as efficient in solving the existing problem as would otherwise be the case.