openapi: Edit `Messages` component to match the `message` event object.

The `Messages` schema present in `#/components/schemas` was a
combination of all keys possible in any message object used in Zulip.
Edit it so that the original `Messages` contains just the keys present
in the `message` event. Also make another schema  `GetMessages` which
adds a few other keys which are received when using `GET /messages`.
The message object in the `/zulip-outgoing-webhook` has also been
modified and corrected.

zerver/openapi/zulip.yaml

@@ -613,7 +613,7 @@ paths:
                           an array of `message` objects, each containing the following
                           fields:
                         items:
-                          $ref: "#/components/schemas/Messages"
+                          $ref: "#/components/schemas/GetMessages"
                   - example:
                       {
                         "anchor": 21,
@@ -4109,6 +4109,11 @@ paths:
                           A dict containing details on the message that triggered the
                           outgoing webhook, in the format used by [`GET /messages`](/api/get-messages).
                       - $ref: "#/components/schemas/Messages"
+                      - properties:
+                          rendered_content:
+                            type: string
+                            description: |
+                              The content/body of the message rendered in HTML.
                   bot_email:
                     type: string
                     description: |
@@ -4400,10 +4405,6 @@ components:
           type: string
           description: |
             The content/body of the message.
-        rendered_content:
-          type: string
-          description: |
-            The content/body of the message rendered in HTML.
         content_type:
           type: string
           description: |
@@ -4438,12 +4439,6 @@ components:
             Data on the recipient of the message;
             either the name of a stream or a dictionary containing basic data on
             the users who received the message.
-        flags:
-          type: array
-          description: |
-            The user's [message flags][message-flags] for the message.
-          items:
-            type: string
         id:
           type: integer
           description: |
@@ -4581,15 +4576,25 @@ components:
           description: |
             The UNIX timestamp for when the message was sent,
             in UTC seconds.
+        type:
+          type: string
+          description: |
+            The type of the message: `stream` or `private`.
+    GetMessages:
+      allOf:
+        - $ref: "#/components/schemas/Messages"
+        - properties:
+            flags:
+              type: array
+              description: |
+                The user's [message flags][message-flags] for the message.
+              items:
+                type: string
             last_edit_timestamp:
               type: integer
               description: |
                 The UNIX timestamp for when the message was last edited,
                 in UTC seconds.
-        type:
-          type: string
-          description: |
-            The type of the message: `stream` or `private`.
             match_content:
               type: string
               description: |