Add type as required property in source object of webhook (#797)

line/line-openapi#103

In the Messaging API webhook, we use `discriminator` to differentiate
classes based on the value of `type`. During the development of bot SDK
for Ruby v2, it seems that only `Source` does not have `type` marked as
required. This is simply a mistake. The type should always be included
to distinguish the kind of JSON, and there should not be any instances
where it is absent.

Co-authored-by: github-actions <[email protected]>

Author: github-actions[bot]

line-openapi

@@ -19,15 +19,15 @@
 import linebot.v3.webhooks.models
 
 
-from typing import Optional, Union
+from typing import Union
 from pydantic.v1 import BaseModel, Field, StrictStr
 
 class Source(BaseModel):
     """
     the source of the event.
     https://developers.line.biz/en/reference/messaging-api/#source-user
     """
-    type: Optional[StrictStr] = Field(None, description="source type")
+    type: StrictStr = Field(..., description="source type")
 
     __properties = ["type"]