Improve how we determine types for fields that have resolvers

[patrick91]

This was prompted by this issue: https://github.com/strawberry-graphql/strawberry/issues/1887

Currently there are some cases where the behaviour we have might not be what one expects. And we could also improve how we handled cases where field type and resolver type don't match.

Based on this type:

def resolver() -> another_type:
    ...

@strawberry.type
class Example:
     field: a_type

Here's a table of the current behaviours

field type	resolver return type	current behaviour	expected behaviour	current type	expected type
int	TypeVar("T")	no error	no error [1]	int	int
int	int	no errors, no warnings	no errors, no warnings	int	int
int	missing	no errors, no warning	no error, maybe a (opt-out) warning?	int	int
int	str	no errors, no warnings	error	str	int

[1] recently fixed here: https://github.com/strawberry-graphql/strawberry/pull/1891

[invokermain]

A side note on #1891: there's actually a few missing cases that the previous PR misses. That PR only fixes the most simple case of the resolver returning TypeVar("T"). My fault for missing the more complex cases. I have opened another issue and PR for this with some discussion questions there: https://github.com/strawberry-graphql/strawberry/issues/1899

I've noticed a separate issue with StrawberryList and type hinted resolvers that return list or List, but that has a separate mechanism, issue here #1901