tsp-openapi3 - warn when encountering unsupported OpenAPI features

### Clear and concise description of the problem

When an OpenAPI file has a field that is unsupported by TypeSpec (as indicated in [the docs](https://typespec.io/docs/getting-started/typespec-for-openapi-dev)), warn during the compilation process.

For example, the `examples` field is used extensively in the [Sefaria API](https://github.com/Sefaria/Sefaria-Project/blob/6faae1e92f428c05daae64d256b891f2e055dd2e/docs/openAPI.json). 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](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [X] Read the [docs](https://typespec.io/docs/).
- [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.