Add a default value for PageSize when listing entries

[sorindumitru]

Ran into the following error when calling ListEntries without a page size:

rpc error: code = ResourceExhausted desc = grpc: received message larger than max (5064719 vs. 4194304)

It would be good if we could specify a default page size if pagination is not requested. The API says that the server can do that. Still worth considering the risks of breaking users with such a change, maybe we can start by warning that the change will be done soon?

We should probably also define some kind of maximum, derived in someway from the maximum message size.