Askari::RegionObserver::Main is a module for top-level DSL usage.

When extended onto the top-level, it will do two things:
 * Offer the coprocessor DSL at the top-level, similar to
   `extend Askari::RegionObserver::DSL`.
 * Unlike extending the DSL, the built `Application` will still reside
   in `Askari::RegionObserver::Main`.

By keeping the `Application` in `Main`, we restore symmetry with other
modules that include the DSL to define coprocessors.  This opens the
way to more easily allowing those modules to also use support methods.

In particular, `Runner` will soon accept modules, not just
`Application`s.  The modules will be mixed into `ExecutionContext`s
allowing helper methods to be used.  Coprocessors defined at the
top-level will use `Main` as their module.