Simplify stage extrapolation

Small GraphQL improvements

* Use connection_type for an array of objects
* Update GraphQL description for job need name

Remove include_merged_yaml parameter

Removes the include_merged_yaml parameter to make code more idiomatic.
The parameter isn't stored, it's computed dynamically when accessed so
this introduces a slight performance overhead, but it's far outweighed
by the pro of having cleaner code and less technical debt.

Stub YamlProcessor response in resolver specs

Refactor relation connection types

* Move request type tests to request specs
* Use proper status enum values

Simplify Ci::Config GraphQL resolver