docs: Add requests.auth code sample

docs/code_samples.md

@@ -255,6 +255,31 @@ class CachedAuthStream(RESTStream):
         return APIAuthenticatorBase(stream=self)
 ```
 
+### Use one of `requests`'s built-in authenticators
+
+```python
+from requests.auth import HTTPDigestAuth
+from singer_sdk.streams import RESTStream
+
+class DigestAuthStream(RESTStream):
+    """A stream with digest authentication."""
+
+    @property
+    def authenticator(self) -> HTTPDigestAuth:
+        """Stream authenticator."""
+        return HTTPDigestAuth(
+            username=self.config["username"],
+            password=self.config["password"],
+        )
+```
+
+`HTTPBasicAuth` and `HTTPProxyAuth` area also available in `requests.auth`. The community
+has also published a few packages with custom authenticators. For example:
+
+- [`requests-aws4auth`](https://github.com/meltano/sdk/issues/1105#:~:text=tedder/requests%2Daws4auth): AWS v4 authentication
+- [`requests_auth`](https://github.com/Colin-b/requests_auth): A collection of authenticators
+  for various services and protocols including Azure, Okta and NTLM.
+
 ### Custom response validation
 
 Some APIs deviate from HTTP status codes to report failures. For those cases,