Make Resolver type definition more accurate

[rafalp]

Our current type for Resolver is Callable[..., Any], catching any arguments and relasing anything.
This is because its currently impossible to tpe Callable that accepts *args or **kwagrs.

This issue is [known to MyPy authors](python/typing#264 but a the time of writing no solution is available.

This is related to #79

[pawelrubin]

This is achievable via Protocol.

@rafalp Let me know if the definitions below would be correct.

class Resolver(Protocol):
    def __call__(self, obj: Any, info: GraphQLResolveInfo, **kwargs: Any) -> Any:
        pass


class TypeResolver(Protocol):
    def __call__(self, obj: Any, *_: Any) -> Optional[str]:
        pass

If so, I'd love to make a contribution.

EDIT
There are already some decent type definitions in graphql-core.

# Note: Contrary to the Javascript implementation of GraphQLFieldResolver,
# the context is passed as part of the GraphQLResolveInfo and any arguments
# are passed individually as keyword arguments.
GraphQLFieldResolverWithoutArgs = Callable[[Any, GraphQLResolveInfo], Any]
# Unfortunately there is currently no syntax to indicate optional or keyword
# arguments in Python, so we also allow any other Callable as a workaround:
GraphQLFieldResolver = Callable[..., Any]

# Note: Contrary to the Javascript implementation of GraphQLTypeResolver,
# the context is passed as part of the GraphQLResolveInfo:
GraphQLTypeResolver = Callable[
    [Any, GraphQLResolveInfo, "GraphQLAbstractType"],
    AwaitableOrValue[Optional[Union["GraphQLObjectType", str]]],
]

However, GraphQLFieldResolver could also be more accurate.

Should ariadne import types from graphql-core or use its own definitions?

Anyway, the Resolver type definition could be rewritten to:

class Resolver(Protocol):
    def __call__(self, obj: Any, info: GraphQLResolveInfo, **kwargs: Any) -> Any:
        pass

[rafalp]

We could use type definitions from graphql-core and and run extra checks on our side at type's definition time.