Add schema validation

[AlexSkrypnyk]

Is your feature request related to a problem? Please describe.
Newer versions of Ahoy may bring new config options, but there is no validation currently in place. This means that some of the options that affect how other existing options work, would be ignored.

In my specific case, 2.2 had the optional added to import, which changed how import operates: it no longer treated the specified file as required if optional was set to true. On the older versions, the optional was not considerd as a part of schema and simply ignored. This lead to Ahoy erroring that my optional file is not available, which confused the user thinking that there is something with the .ahoy.yml file having an error. Had there be a validation of config schema, Ahoy would issue a warning that the newer schema in .ahoy.yml with optional is not valid, which would prompt the user to look into updating Ahoy.

Describe the solution you'd like
It would be great to validate the config schema and issue a warning to user that they are either using an incorrect schema or should upgrade Ahoy.

Additional context
#1551 ahoy throws error after running starter script

[ocean]

@AlexSkrypnyk I've created a PR with some validation and also an ahoy doctor command which allows people to test their files and potentially will tell them to check their ahoy version if it appears out of date etc, have a look at #154