make_executable_schema resolvers arg should accept dict of dicts or list of dicts of dicts

[rafalp]

The make_executable_schema utility should optionally take list of dicts of dicts (AKA "resolvers map"), this would allow larger projects to easily split and compose resolvers as needed:

from ariadne import make_executable_schema

from products.graphql import resolvers as products_resolvers
from users.graphql import resolvers as users_resolvers


typedefs = "..."
resolvers = [products_resolvers, users_resolvers]

schema = make_executable_schema(typedefs, resolvers)

This task will likely require #13 to be done first, so we are 100% certain that all resolver mappings are dicts.