[3.8] Add optional parameter to 'searchForMedia' to control which subfolders are traversed

[solth]

This pull request adds a new optional paramter subfolders to KitodoScript searchForMedia. By setting this parameter to the value configured the project folder settings are taken into account. This restricts the script to those process subfolders whose contents are set to be linked in the meta.xml file via the project folder settings. Without this parameter (or setting it to any other value than "configured") retains the current behavior where the contents of all process subfolders are linked in the meta.xml file when calling the script searchForMedia.

Thus fixes #6862 for Kitodo 3.8.x (contrary to recent announcements this means there will probably be at least one more bugfix release for version 3.8 of Kitodo)

[henning-gerhardt]

Code changes looks good but I have the problem that I can not say if this changes are working or not.

We have 3 directories which are all of them are defined to not be linked in the mets file but they are in the internal mets file references as they are used a generator source, preview and media view.

Must I define a additional fourth directory in local system and as a folder with the "no mets link" attribute to see the difference?

And additional question: I though that this linking is only relevant for exporting the process and the meta data / creating additional file groups in the export data - so linking is also relevant for internal meta data file usage?