support sa_key from string

Author: uzhastik

ydb/iam/auth.py

@@ -99,14 +99,25 @@ def set_token_expiration_timeout(self, value):
     @classmethod
     def from_file(cls, key_file, iam_endpoint=None, iam_channel_credentials=None):
         with open(os.path.expanduser(key_file), "r") as r:
-            output = json.loads(r.read())
-        account_id = output.get("service_account_id", None)
+            key = r.read()
+
+        return BaseJWTCredentials.from_content(
+            cls,
+            key,
+            iam_endpoint=iam_endpoint,
+            iam_channel_credentials=iam_channel_credentials
+        )
+
+    @classmethod
+    def from_content(cls, key, iam_endpoint=None, iam_channel_credentials=None):
+        key_json = json.loads(key)
+        account_id = key_json.get("service_account_id", None)
         if account_id is None:
-            account_id = output.get("user_account_id", None)
+            account_id = key_json.get("user_account_id", None)
         return cls(
             account_id,
-            output["id"],
-            output["private_key"],
+            key_json["id"],
+            key_json["private_key"],
             iam_endpoint=iam_endpoint,
             iam_channel_credentials=iam_channel_credentials,
         )