This refactoring aims to ease testability and clean up a lot of soft touchy
typings in test code.
This is the first step of this refactoring introducing the concept and
implementing missing interfaces and stubs. At the end of the day,
ServerVariablesHandler should completely disappear and every variable should
be injected in the endpoint handler builder itself.
Client and server now have their own tsconfig so that the transpilation is only
done on the part that is being modified.
It also allows faster transpilation since tests are now excluded from tsconfig.
They are compiled by ts-node during unit tests execution.
