Align JsonSchema and OpenAPI extension decorators

[chrisradek]

Currently the JsonSchema and OpenAPI @extension decorators have different behavior. Given Open API 3.1 is based on JsonSchema, the @extension decorator should work the same between those 2 emitters. This may also extend to the Open API 3.0 emitter.

[markcowl]

• break when a model type is passed in at rc0 (or breaking change release), allow model input as schemas later

[allenjzhang]

Moving to Post 1.0. Tentative sprint: July 25.