Fix Execution.action_group type and add missing fields

action_group uses _to_optional which can return None, but the type
annotation was non-optional. Also add start_time, execution_type and
execution_sub_type which are present in the API response but were
silently dropped by _flexible_init.

Author: iMicknl

pyoverkiz/models.py

@@ -653,7 +653,16 @@ class Execution:
     description: str
     owner: str = field(repr=obfuscate_email)
     state: str
-    action_group: ActionGroup = field(converter=_to_optional("ActionGroup"))
+    action_group: ActionGroup | None = field(
+        default=None, converter=_to_optional("ActionGroup")
+    )
+    start_time: int | None = None
+    execution_type: ExecutionType | None = field(
+        default=None, converter=_to_optional_enum(ExecutionType)
+    )
+    execution_sub_type: ExecutionSubType | None = field(
+        default=None, converter=_to_optional_enum(ExecutionSubType)
+    )
 
 
 @_flexible_init