platform: streaming and unary callback ergonomics

Writing network requests with the various callbacks we have to attach specific closures and end of stream checks. This could be cumbersome when a user wants to only make a simple unary request (and not a streaming request). We should revisit how a user would use Envoy in these more common cases

One option is for us to expose an additional unary (or non-streaming) interface and handle all these different cases for the user