Make GraphQL schema language support classic resolver

[mcg-web]

Q	A
Bug report?	no
Feature request?	yes
BC Break report?	no
RFC?	no
Version/Branch	0.12

Right now GraphQL schema language implementation only supports ResolverMap, it should also support classic resolver.

[IgorSzymanski]

Would love this to be implemented as working with .yaml is honestly awful. No autocompletion, no nothing in IDE.

[murtukov]

@mcg-web What is classic resolver? Could you provide an example?