[http-specs]: Client level definition in unbranded scenario

To be more specific, this [file](https://github.com/Azure/cadl-ranch/blob/main/packages/cadl-ranch-specs/http/type/property/nullable/client.tsp) in previous cadl-ranch is deleted from this [case](https://github.com/microsoft/typespec/tree/main/packages/http-specs/specs/type/property/nullable) in new http-specs. I could understand it is because this client.tsp contains `@convenientAPI` from TCGC. But the issue here is unbranded generator should have a way to customize convenience API.

Further, all the decorators in TCGC should have a right representation in unbranded scenario and then all of the client.tsp could be added into http-specs.