Add default Validation Rule(s) for schema reference validity

[martincostello]

Is your feature request related to a problem? Please describe.

I found a bug in ASP.NET Core where invalid schema references were being generated that are circular references: dotnet/aspnetcore#63090

I looked to add a test that uses the validation rules, but the built-in default rules do not detect any such issues (though they found others).

editor.swagger.io does detect these issues, as that's how I found the bug.

Describe the solution you'd like

One or more validation rules that verify that OpenAPI schema references are valid.

Describe alternatives you've considered

Write code to manually traverse the document and validate schema references (e.g. dotnet/aspnetcore@06ffc01).

Additional context

dotnet/aspnetcore#63090