Edit Pipelines API annotation

grape-swagger only takes the first type in the types array because OpenAPI V2 does not support multiple types. As such, the current OpenAPI output would only define the singular String type for the scope parameter. Since the parameter already uses coerce_with to coerce any String inputs into Array[String], the String type annotation is redundant and we can simply use Array[String] only.

This also adds the missing nested array parameters for variables.

The OpenAPI V2 documentation will be used for dogfooding API fuzzing as per #372422 (closed) and the OpenAPI autogeneration &8926.