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

[pshao25]

To be more specific, this file in previous cadl-ranch is deleted from this case 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.