Add type as required property in source object of webhook (#1235)

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]

lib/webhook/model/source.ts

@@ -28,5 +28,5 @@ export type SourceBase = {
    *
    * @see <a href="https://developers.line.biz/en/reference/messaging-api/#source-user">type Documentation</a>
    */
-  type?: string /**/;
+  type: string /**/;
 };