Implement merging of dynamic fields on ticket merge

For a list of configured dynamic fields, in case of a ticket merge, the value from the merge ticket is copied into the main ticket if the main ticket has no value set.

This feature has been requested at http://bugs.otrs.org/show_bug.cgi?id=10191 / https://otrsteam.ideascale.com/a/dtd/Merge-dynamic-and-free-fields-when-merging-ticets/461538-10369 and we already use a less generic implementation internally.