Fix interpolation to structured config from within typed list

This PR closes #1005, fixing a bug where a validation error was raised
when an interpolation points to a structured config from within a typed
list.

```python
from dataclasses import dataclass, field
from omegaconf import MISSING, OmegaConf

@DataClass
class User:
    name: str = MISSING

@DataClass
class Config:
    user: User = User("John")
    users: list[User] = field(default_factory=lambda: ["${user}"])

cfg = OmegaConf.structured(Config)
OmegaConf.resolve(cfg)
print(cfg)
```
BEFORE
------
```text
$ python repro.py
$ python repro.py
Traceback (most recent call last):
  ...
omegaconf.errors.ValidationError: Invalid type assigned: str is not a subclass of User. value: ${user}
    full_key: users[0]
    reference_type=List[User]
    object_type=list
```
AFTER
-----
```text
$ python repro.py
{'user': {'name': 'John'}, 'users': [{'name': 'John'}]}
```

omegaconf/listconfig.py

@@ -106,6 +106,8 @@ def _validate_set(self, key: Any, value: Any) -> None:
         vk = get_value_kind(value)
         if vk == ValueKind.MANDATORY_MISSING:
             return
+        if vk == ValueKind.INTERPOLATION:
+            return
         else:
             is_optional, target_type = _resolve_optional(self._metadata.element_type)
             value_type = OmegaConf.get_type(value)

tests/structured_conf/data/attr_classes.py

@@ -76,6 +76,8 @@ class OptionalUser:
 class InterpolationToUser:
     user: User = User("Bond", 7)
     admin: User = II("user")
+    admin_list: List[User] = [II("user")]
+    admin_dict: Dict[str, User] = {"bond": II("user")}
 
 
 @attr.s(auto_attribs=True)

tests/structured_conf/data/dataclasses.py

@@ -77,6 +77,8 @@ class OptionalUser:
 class InterpolationToUser:
     user: User = field(default_factory=lambda: User("Bond", 7))
     admin: User = II("user")
+    admin_list: List[User] = field(default_factory=lambda: [II("user")])
+    admin_dict: Dict[str, User] = field(default_factory=lambda: {"bond": II("user")})
 
 
 @dataclass

tests/structured_conf/data/dataclasses_pre_311.py

@@ -77,6 +77,8 @@ class OptionalUser:
 class InterpolationToUser:
     user: User = User("Bond", 7)
     admin: User = II("user")
+    admin_list: List[User] = field(default_factory=lambda: [II("user")])
+    admin_dict: Dict[str, User] = field(default_factory=lambda: {"bond": II("user")})
 
 
 @dataclass

tests/structured_conf/test_structured_basic.py

@@ -247,6 +247,10 @@ def test_interpolation_to_structured(self, module: Any, resolve: bool) -> None:
             OmegaConf.resolve(cfg)
         assert OmegaConf.get_type(cfg.admin) is module.User
         assert cfg.admin == {"name": "Bond", "age": 7}
+        assert OmegaConf.get_type(cfg.admin_list[0]) is module.User
+        assert cfg.admin_list == [{"name": "Bond", "age": 7}]
+        assert OmegaConf.get_type(cfg.admin_dict["bond"]) is module.User
+        assert cfg.admin_dict == {"bond": {"name": "Bond", "age": 7}}
 
     class TestMissing:
         def test_missing1(self, module: Any) -> None: