Implement an 'addins' feature to copy in files to the root of the springboot jar

[plaird]

There is an Salesforce internal use case that asks for a static file to be copied into the root of the springboot jar. It is not important what the use case is. Assume it is a license file, but it could be anything.

Add an attribute to the springboot rule such that a list of files (or a filegroup) can be specified.

addins = [":info.txt", ":author.txt"],

Such that they appear in the generated springboot jar:

jar -tvf bazel-bin/examples/demoapp/demoapp.jar | grep .txt
   132 Fri Jan 01 00:00:00 PST 2010 info.txt
    11 Fri Jan 01 00:00:00 PST 2010 author.txt