[MSOURCES-106] Allow inclusion of additional files like LICENSE and NOTICE in source jar

**[Luc Maisonobe](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=luc)** opened **[MSOURCES-106](https://issues.apache.org/jira/browse/MSOURCES-106?redirect=false)** and commented

The source jar, despite not being intended as a full-featured source distribution, can be considered a derivative work. Some licenses (like the Apache License!) require that derivative work includes copies of important legal files, like LICENSE and NOTICE in the case of the
Apache License, but there may be other cases.

Currently, there are no provisions in the source plugin to add files that are not within
the source tree. Typical Apache projects put the LICENSE and NOTICE at the top level
of the project, outside of the source tree.

It would be nice to have an optional parameter "additionnalFiles" in the various goals source:jar, source:jar-no-fork, source:test-jar, source:tst-jar-no-fork where a list of additional files could be put in the jar (putting them at the top level of the jar would probably be OK, but some other plugins put such files in the META-INF directory within the jar)

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**Affects:** 3.0.1