fix: Improve Accept.best_match typing (#4487)

* Improve `Accept.best_match` typing

It used to be this way: 

```python
.best_match(['application/json'], default='application/json')
# -> reveals `Optional[str]` as the return type
```

However, default is `str` and this is a typing error.

Now it works as expected when passing `best_match(['application/json'], default='application/json')` and reveals `str` as the return type. Full testing is here: https://mypy-play.net/?mypy=latest&python=3.12&flags=strict&gist=bf3a081c25f31ddfd2da6614fb59deff

* Update headers.py

* Update headers.py

* Update headers.py

Author: sobolevn

litestar/datastructures/headers.py

@@ -12,6 +12,7 @@
     Optional,
     Union,
     cast,
+    overload,
 )
 
 import msgspec
@@ -440,6 +441,12 @@ def __getitem__(self, key: int) -> str:
     def __iter__(self) -> Iterator[str]:
         return map(str, self._accepted_types)
 
+    @overload
+    def best_match(self, provided_types: list[str], default: None = None) -> Optional[str]: ...
+
+    @overload
+    def best_match(self, provided_types: list[str], default: str) -> str: ...
+
     def best_match(self, provided_types: list[str], default: Optional[str] = None) -> Optional[str]:
         """Find the best matching media type for the request.