DatabaseClient: add missing class methods and reflect Django 3.2 refactor (#846)

* DatabaseClient: add missing class method `settings_to_cmd_args_env` and add missing arg to `runshell`

The `DatabaseClient.runshell()` was refactored in Django 3.2 to be more standardized
across the different database backends.

* DatabaseClient: make `settings_dict` dict type better by specifying key/value types

* tests: add missing db backend client files to `import_all_modules`

django-stubs/db/backends/oracle/client.pyi

@@ -1,5 +1,13 @@
+from typing import Any, Dict, Iterable, List, Tuple
+
 from django.db.backends.base.client import BaseDatabaseClient as BaseDatabaseClient
 
 class DatabaseClient(BaseDatabaseClient):
     executable_name: str = ...
     wrapper_name: str = ...
+    @staticmethod
+    def connect_string(settings_dict: Dict[str, Any]) -> str: ...
+    @classmethod
+    def settings_to_cmd_args_env(
+        self, settings_dict: Dict[str, Any], parameters: Iterable[str]
+    ) -> Tuple[List[str], None]: ...