Investigate internal streaming RPC overhead for List methods

[SanjayVas]

Originally brought up by @tristanvuong2021.

We may be paying an unnecessarily penalty for our use of server streaming for the internal service methods that support public List methods. Server streaming has extra communication overhead for each response. The response type for these is the entity that maps to the public API resource, meaning there is one RPC response per resource. If the only caller is a List method, then we're not gaining any benefits of streaming as the result will be fully collected in order to build the public API response.

Effectively using the resource as our internal response type also means that we have to request one extra response from the internal service in order to determine whether to set the next_page_token field in the public response.