Introduce check concept from "Azure/openapi-diff" project

The project Azure/openapi-diff produce following output when two versions of a spec are compared: a list of items (error, warning, info) is computed.

Example:

$ oad compare old/added_path.json new/added_path.json
{
  "id": "1001",
  "code": "NoVersionChange",
  "message": "The versions have not changed.",
  "jsonref": "#",
  "json-path": "#",
  "type": "Info"
}
{
  "id": "1039",
  "code": "AddedOperation",
  "message": "The new version is adding an operation that was not found in the old version.",
  "jsonref": "#/paths/~1api~1Operations/post",
  "json-path": "#/paths/api/Operations/post",
  "type": "Info"
}
{
  "id": "1038",
  "code": "AddedPath",
  "message": "The new version is adding a path that was not found in the old version.",
  "jsonref": "#/paths/~1api~1Paths",
  "json-path": "#/paths/api/Paths",
  "type": "Info"
}

The list of rules that are currently checked can be found in their git repository: docs
I had a look at their test suite in this folder: Swagger

To run their the oad tool against it, I had to perform some changes: https://github.com/Azure/openapi-diff/pull/108
Then I have converted the old Swagger 2.0 format to OpenAPI 3.0.1 format.

Of course, I will share my work.
We can then continue the discussion and see how this could be integrated.

Beginne mit der Lektüre der Azure/openapi-diff-Dokumentation und der openapi-diff/src/modeler/AutoRest.Swagger.Tests/Resource/Swagger-Tests und vergleiche anschließend deren gemeldete Checks mit dem aktuellen Vergleichsverhalten dieses Projekts. Das Issue nennt keine lokalen Dateien oder keinen Einstiegspunkt; für den Abschluss wären ein vereinbarter Integrationsumfang und entsprechende Checks erforderlich.