Describe the bug
There is a feature of switching the CSV seperator in the mass import page to match the column seperator of the imported csv file.

When using this feature the list with the metadata keys (representing the current column headings) is recreated using List.of

Using List.of here creates a problem because the new list is immutable so no new items can be added to the list. When using the "Add Metadata" feature

afterwards an exception is thrown because Kitodo tries to add items to an now immutable list.

https://github.com/kitodo/kitodo-production/blob/83b1b9fcbb586add543a55c8d7844d0d82077e36/Kitodo/src/main/java/org/kitodo/production/forms/massimport/AddMetadataDialog.java#L116-117

To Reproduce
Steps to reproduce the behavior:

1. Upload a csv file seperated by comma (",")
2. Switch the seperator to from semicolon to comma (";")
3. Try to add metadata

Expected behavior
The list with the metadata keys probably has to stay mutable after changing the csv separator.

Release
Kitodo Production 3.6.2, Master