Showcase should test the `json_name` proto annotation

[diegomarquezp]

Thanks Diego! As a follow up, can we create an issue in showcase to support json_name?

Originally posted by @blakeli0 in #12940 (review)

Thanks Diego! As a follow up, can we create an issue in showcase to support json_name?

#12940 introduced support for the json_name annotation.

We must also ensure that showcase reflects query, path and body parameters using this annotation.