Create route to get json schema to validate config

Fix visibility
Make compatible with php 7.2
Remove returing instance
Increase coverage
Reduce psalm info
Throw exception if system not defined
Increment coverage

Signed-off-by: Vitor Mattos <vitor@php.rio>

appinfo/routes.php

@@ -91,6 +91,7 @@ return [
 		['name' => 'board_api#updateAcl', 'url' => '/api/v{apiVersion}/boards/{boardId}/acl/{aclId}', 'verb' => 'PUT'],
 
 		['name' => 'board_import_api#getAllowedSystems', 'url' => '/api/v{apiVersion}/boards/import/getSystems','verb' => 'GET'],
+		['name' => 'board_import_api#getConfigSchema', 'url' => '/api/v{apiVersion}/boards/import/config/schema/{name}','verb' => 'GET'],
 		['name' => 'board_import_api#import', 'url' => '/api/v{apiVersion}/boards/import','verb' => 'POST'],