Create sbt plugin for header check?

So I just realized that we probably have to abstract all of the `sbt-header-check` logic that is done for checking the Apache copyright headers into its own sbt plugin because in the current setup, while it does correctly check for the sources that are part of the main project, it doesn’t check for the sources that are extending the sbt build (i.e. `*.scala` files in `project/`).

Theoretically to solve this you would have to add do `addSbtPlugin("de.heikoseeberger" % "sbt-header" % "5.7.0")` in `project/project/plugins.sbt` (take note of the additional nested `project` folder) so that the plugin applies to the sbt source files however we would also have to redefine all of the logic in `CopyrightHeader.scala` (hence the need to put it into a sbt plugin).

@pjfanning What do you think?