registration: Refactor validation logic to avoid duplication

registration: Refactor validation logic to avoid duplication

Previously, logic to validate extension.json files was in two places:
validateRegistrationFile.php maintenance script, and the
ExtensionJsonValidationTest.php structure test. This caused duplication
as validation became more complex (e.g. usage of spdx-licenses library).

A generic ExtensionJsonValidator class now handles most of the
validation work, while the maintenance script and test case just wrap
around it for their output formats.

Change-Id: I47062a4ae19c58ee1b1f2bb4877913259bf19c8b