Fix #756: Improve server codegen by leveraging RuntimeType (#774)

* Fix SdkBody test to just use assert!

* Fixes #756: Improve server codegen by leveraging RuntimeType. Cleanup
server CodegenVisitor to only render protocol specific code for
OperationShapes.

* Fix silly error I introduced in the smithy-http SdkBody tests

* Apply suggestions from code review

Co-authored-by: david-perez <d@vidp.dev>

* Extract Json ser/de into an interface. Refactor the CodegenVisitor
structureShape to generate both inputs and outputs outside of the
ProtocolGenerator.

* Gather error members from the HTTP index

* Use operationShape as input for server generation of input/output/error

Co-authored-by: david-perez <d@vidp.dev>