streaming interface for paging calls

[alexander-fenster]

Who knew we had this?

https://github.com/googleapis/gapic-generator/blob/e3501faea84f61be2f59bd49a7740a486a02fa6b/src/main/resources/com/google/api/codegen/nodejs/main.snip#L477-L508

Since the generated code does not have unit tests for this, we can also skip testing it, but it makes sense to try adding it to the test showcase applications (both JS and TS).