chore: Add warnings regarding consuming externally sourced credentials (#1655)

* chore: Add warnings regarding consuming externally sourced credential configurations

* update syntax

* remove in ADC

* period

* make it warning

* update warning syntax

* update secret after rebase

Author: sai-sunder-s

docs/user-guide.rst

@@ -29,6 +29,17 @@ that supports OpenID Connect (OIDC).
 Obtaining credentials
 ---------------------
 
+.. warning::
+    Important: If you accept a credential configuration (credential JSON/File/Stream)
+    from an external source for authentication to Google Cloud Platform, you must
+    validate it before providing it to any Google API or client library. Providing an
+    unvalidated credential configuration to Google APIs or libraries can compromise
+    the security of your systems and data. For more information, refer to
+    `Validate credential configurations from external sources`_.
+
+.. _Validate credential configurations from external sources:
+    https://cloud.google.com/docs/authentication/external/externally-sourced-credentials
+
 .. _application-default:
 
 Application default credentials

google/auth/_default.py

@@ -85,6 +85,17 @@ def load_credentials_from_file(
     user credentials, external account credentials, or impersonated service
     account credentials.
 
+    .. warning::
+        Important: If you accept a credential configuration (credential JSON/File/Stream)
+        from an external source for authentication to Google Cloud Platform, you must
+        validate it before providing it to any Google API or client library. Providing an
+        unvalidated credential configuration to Google APIs or libraries can compromise
+        the security of your systems and data. For more information, refer to
+        `Validate credential configurations from external sources`_.
+
+        .. _Validate credential configurations from external sources:
+            https://cloud.google.com/docs/authentication/external/externally-sourced-credentials
+
     Args:
         filename (str): The full path to the credentials file.
         scopes (Optional[Sequence[str]]): The list of scopes for the credentials. If
@@ -137,6 +148,17 @@ def load_credentials_from_dict(
     user credentials, external account credentials, or impersonated service
     account credentials.
 
+    .. warning::
+        Important: If you accept a credential configuration (credential JSON/File/Stream)
+        from an external source for authentication to Google Cloud Platform, you must
+        validate it before providing it to any Google API or client library. Providing an
+        unvalidated credential configuration to Google APIs or libraries can compromise
+        the security of your systems and data. For more information, refer to
+        `Validate credential configurations from external sources`_.
+
+    .. _Validate credential configurations from external sources:
+        https://cloud.google.com/docs/authentication/external/externally-sourced-credentials
+
     Args:
         info (Dict[str, Any]): A dict object containing the credentials
         scopes (Optional[Sequence[str]]): The list of scopes for the credentials. If