Windows support for Spring Boot rule

[plaird]

Currently, with a bash implementation, the rule is not portable natively to Windows. It seems like there is weak support for running Bazel from cygwin, but that isn't a real solution.

https://docs.bazel.build/versions/master/windows.html#bazel-build-without-bash

There is already an issue #3 to reimplement the bash script in another language for maintainability reasons. Please track the bash migration discussions there.

This issue tracks testing, and fixing any other issues found while running this rule on Windows.