interpreterbase/decorators: KwargIfno validator must take an relaxed type

I've used any because it needs to be infinitely recursive, something
that we simply can't model. But basically until it goes into validator
we have no way of knowing what's going on, since one can write code
like:

```python
KwargInfo[str]('arg', object, validator=_some_very_complex_logic_to_get_specific_string)
```

As such, we can't assume that validator is receiving a type _T, it could
be anything.

mesonbuild/interpreterbase/decorators.py

@@ -373,7 +373,7 @@ class KwargInfo(T.Generic[_T]):
                  since_values: T.Optional[T.Dict[str, str]] = None,
                  deprecated: T.Optional[str] = None,
                  deprecated_values: T.Optional[T.Dict[str, str]] = None,
-                 validator: T.Optional[T.Callable[[_T], T.Optional[str]]] = None,
+                 validator: T.Optional[T.Callable[[T.Any], T.Optional[str]]] = None,
                  convertor: T.Optional[T.Callable[[_T], object]] = None,
                  not_set_warning: T.Optional[str] = None):
         self.name = name