Create Preparation interface

[mehulkaklotar]

Preparations are setup steps that should run ahead of checks. This will allow any setup work required before a check can be run to take place. Each preparation will handle the logic required to prepare the environment for the checks and return a clean up function to be run after the checks are complete.

To provide a consistent API across the infrastructure, a Preparation interface will be created. This will define a single prepare method that any class should use to define the setup logic required.

Acceptance Criteria

• A new Preparation interface exists in includes/Checker/Preparation.php
• The Preparation interface contains a single public prepare() method
• The prepare() method accepts no parameters
• An Exception will be thrown if the prepare() method fails
• The prepare() method returns a closure

[felixarntz]

@jjgrainger ACs LGTM, though we may want to add a point to also document that the prepare() method returns a closure to call for clean up.