Authorization should allow to override, such as request_token() metho…

…ds, there are some customizing authorization flows don't support to get access_token thought local client, even if using environment variables are not allowed. More reference: https://open.feishu.cn/document/common-capabilities/sso/api/get-access_token?lang=en-US (flet-dev#776)

sdk/python/flet/page.py

@@ -364,8 +364,9 @@ def login(
         on_open_authorization_url=None,
         complete_page_html: Optional[str] = None,
         redirect_to_page=False,
+        authorization=Authorization
     ):
-        self.__authorization = Authorization(
+        self.__authorization = authorization(
             provider,
             fetch_user=fetch_user,
             fetch_groups=fetch_groups,