requests: Add connection property (type HTTPAdapter) to the Response class (#12279)

The `Response` instance is built by `HTTPAdapter`. When built, a property called `connection` is added to the `Response` that points back to the `HTTPAdapter` that created it. For example, this is used in the `requests` library's `HTTPDigestAuth` class to remake requests with credentials after authorization is requested by a server.

stubs/requests/requests/models.pyi

@@ -8,6 +8,7 @@ from typing_extensions import Self
 from urllib3 import exceptions as urllib3_exceptions, fields, filepost, util
 
 from . import auth, cookies, exceptions, hooks, status_codes, utils
+from .adapters import HTTPAdapter
 from .cookies import RequestsCookieJar
 from .structures import CaseInsensitiveDict as CaseInsensitiveDict
 
@@ -123,6 +124,7 @@ class Response:
     cookies: RequestsCookieJar
     elapsed: datetime.timedelta
     request: PreparedRequest
+    connection: HTTPAdapter
     def __init__(self) -> None: ...
     def __bool__(self) -> bool: ...
     def __nonzero__(self) -> bool: ...