tsp-openapi3 - warn when encountering unsupported OpenAPI features

[Arithmomaniac]

Clear and concise description of the problem

When an OpenAPI file has a field that is unsupported by TypeSpec (as indicated in the docs), warn during the compilation process.

For example, the examples field is used extensively in the Sefaria API. Running tsp-openapi3 should log a warning message for each examples field encountered (until, of course, #2700 is implemented.)

Checklist

• [X] Follow our Code of Conduct
• [X] Read the docs.
• [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[Arithmomaniac]

On the flop side, it should also warn when OpenAPI features are missing too, like operationId, but perhaps that should be addressed in a separate issue.

[markcowl]

est: 3