openapi/json-schema `@extension` alignment - remove x- constraint and support values

[chrisradek]

Clear and concise description of the problem

The @typespec/openapi @extension decorator should be updated to behave similarly to the @typespec/json-schema @extension decorator.

This task tracks the (mostly) non-breaking changes to the extension:

1. Removal of the starts with x- extension name constraint
2. Add support for passing in value kinds to generate raw data

Additionally, a warning should be added for passing in types to the @extension as this behavior will change in a subsequent release to emit Open API schemas instead of raw data.

Checklist

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