Add option to reuse licenses.xml

[criztovyl]

Hi there,

using dependency:list to fetch dependencies/licenses can be quite inefficient, this PR adds and option to re-use an already existing licenses.xml file as generated by licenses-maven-plugin:download-licenses.

I am aware it might have been better to open an issue first, but it has been some time since I wrote non-CI code so I did it also for fun. :)

Points to be clarified:

• dependency on license-maven-plugin -- I also considered copying the files in verbartim (it's only three files), but I stuck with the dependency with exclusions
• I hid it behind a feature flag and the feature flag at the same time disables the dependency:list approach, I am open to changes, for the use-case I have in mind the current state does the job. :)
• (unit-)tests