Move title + commit information into CLI:App constructor

Created by: rerpha

https://github.com/ess-dmsc/kafka-to-nexus/blob/cfb0bddf7d28da2c7b5aa4353019e83032b5fd07/src/kafka-to-nexus.cpp#L26

we should move this into the CLI:App constructor underneath and wrap it in a fmt::format() call