Add support to restrict pull requests to defined paths

[AndreiVidrasco]

Would it be possible to restrict building of PR based on the paths that were changed.
This would be useful for monorepos or for e.g. if someone change a documentation file, there is no need to run build.

As a reference same implementation in github pull request builder
jenkinsci/ghprb-plugin#293

[CodeMonk]

That is a decent idea. Labeling as enhancement.