Edit Project Import/Export API annotation

This modifies a documentation-only type override to allow it to be compatible with autogenerated grape-swagger OpenAPI documentation. The standard type ::API::Validations::Types::WorkhorseFile cannot be parsed by grape-swagger and will lead to incorrect OpenAPI output.

It also adds the missing type key in the parameter annotation, otherwise it will be parsed incorrectly by grape-swagger.

Finally, it adds the missing accepted values for the http_method parameter as described in the documentation (https://docs.gitlab.com/ee/api/project_import_export.html#schedule-an-export).

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