* Generate notice file during artifact build
Adjust our existing NOTICE.txt file to be built every time. This ensures
we collect LICENSE and NOTICE information of all bundled Logstash and non-Logstash
dependencies.
Changes:
* if NOTICE file is present in gems, include that text in our own NOTICE.TXT file.
* we also need to include the whole copyright block in NOTICE (meaning copyright + license)
