Validate resolver types against field types

Current we don’t throw errors in cases like this:

def example() -> str:
    ...

@strawberry.type
class Q:
    x: int = strawberry.field(resolver=example)

We should raise an error when this happens. We should also maybe allow to change the error to a warning, to make it easier to upgrade.

We could also prevent this (maybe as opt-in/out behaviour):

def example():  # missing type
    ...

@strawberry.type
class Q:
    x: int = strawberry.field(resolver=example)

[bikeshedder]

I’m generating some of my resolvers dynamically. I don’t define the return type or use Any in this case. If you implement this feature please don’t forget this case as a resolver might return the right type at runtime but is unknown at the time it’s created.

[bikeshedder]

In my code it looks somewhat like that:

def create_resolver(**filter):
    def resolver():
        # XXX In the actual code some DB query or similar is issued.
        return []
    return resolver

@strawberry.type
class Foo:
    bars: List['Bar'] = strawberry.field(resolver=create_resolver(type='bar'))

@strawberry.type
class Bar:
   name: str

In #1449 I added a unit test for that. The PR is about resolvers in different modules, but the test also includes the part of a generic resolver (no type annotation).