Make location of shipkit.gradle configurable

[nipafx]

Many tools (CheckStyle, Spotless, Formatter, Shipkit, Gradle, ...) surrounding a decently sized project require configuration and by default they all read their file from the project's root folder. That leads to a lot of clutter. I'm trying to reduce that by pushing most of those files into a dedicated folder (e.g. .infra). As I see it, Shipkit does not allow that.

Request: Allow configuring the location and name of shipkit.gradle in build.gradle.