Parametrize register function with service class

We want to declare different types of services via a common interface.
Declare the interface for a service
Move common operations to service super class
Code is now optional in services

A passthrough adapter doesn't have code.
Remove code from sub-class
Move to_json to super-class
Declare methods as abstract
Add comment
Remove dead code
Move more code to super-class
First pass for a passthrough service
Remove debug code
Allow POST to passthrough
Add missing function to service interface
Add tests for passthrough service

(1 test failing)
Passthrough accepts url-encoded and regular data