Allow running scalafmt on generated sources

[OlegYch]

I couldn't find a way to hook scalafmt into generated guardrail sources.
Ideally it should run automatically without need for manual configuration, but that's probably sbt or scalafmt responsibility.

[OlegYch]

as a workaround i use Compile / scalafmt / unmanagedSources ++= (Compile / managedSources).value but that breaks caching of generated files

[blast-hardcheese]

I think there's an opportunity to run the transformation before emission, which would solve the caching issue.

I don't have the time to look into this at the moment, but I'm happy to tend the release were a patch to arrive.

Looking though this, I believe the most sensible thing seems to be to create a new Key, perhaps guardrailFormatter, dereferenced on this line:

      scope / Keys.guardrail := cachedGuardrailTask(SbtKeys.name.value, scope.name, SbtKeys.scalaBinaryVersion.value)(name, _root_.sbt.Keys.streams.value)((scope / Keys.guardrailTasks).value, (scope / SbtKeys.managedSourceDirectories).value),

(Pardon for not directly linking the file, GitHub for mobile is quite limited for stuff like this.)

I'd think it could match whatever the type of sbt-scalafmt's corresponding Task, so we could guardrailFormatter := scalafmt or so, but I'm open to other ideas as well.

[OlegYch]

turns out it was easier, submitted #236