Ability to add custom fallback site alias resolvers

[bomoko]

Following from the discussion here drush-ops/drush#5273 (and, sorry it has taken me so long to get to this) - it would be really useful for the site-alias manager to support fallback resolution via events.

The suggestion is that we allow client code to register events with the SAM and if an alias cannot be resolved, we give the client the option of resolving it for themselves.

In the attached POC I've used the Symfony EventDispatcher component to achieve this - but am quite keen to get design/code guidance on this to get something like this into the project.