Make types in `ExAws.Request` public

[benmurden]

The documentation for ExAws.request/2 shows generic return types, but I'd like to be able to use the types in ExAws.Request to write type specs so I get better Dialyzer warnings.

https://hexdocs.pm/ex_aws/ExAws.html#request/2

I could just use them of course, but the fact that they are not documented makes people uneasy.