linkifiers: Extract shared parameter in zulip.yaml.

In zulip.yaml, `pattern` and `url_format_string` are extracted
as a shared parameters.

This is done as a preparatory commit for `Add settings UI to edit
linkifiers`.

Related issue: #10830.

zerver/openapi/zulip.yaml

@@ -6620,26 +6620,8 @@ paths:
 
         `POST {{ api_url }}/v1/realm/filters`
       parameters:
-        - name: pattern
-          in: query
-          description: |
-            The [Python regular
-            expression](https://docs.python.org/3/howto/regex.html) that should
-            trigger the linkifier.
-          schema:
-            type: string
-          example: "#(?P<id>[0-9]+)"
-          required: true
-        - name: url_format_string
-          in: query
-          description: |
-            The URL used for the link. If you used named groups for the `pattern`,
-            you can insert their content here with
-            `%(name_of_the_capturing_group)s`.
-          schema:
-            type: string
-          example: https://github.com/zulip/zulip/issues/%(id)s
-          required: true
+        - $ref: "#/components/parameters/LinkifierPattern"
+        - $ref: "#/components/parameters/LinkifierURLFormatString"
       responses:
         "200":
           description: Success.
@@ -11948,3 +11930,25 @@ components:
           - type: integer
       example: "20"
       required: false
+    LinkifierPattern:
+      name: pattern
+      in: query
+      description: |
+        The [Python regular
+        expression](https://docs.python.org/3/howto/regex.html) that should
+        trigger the linkifier.
+      schema:
+        type: string
+      example: "#(?P<id>[0-9]+)"
+      required: true
+    LinkifierURLFormatString:
+      name: url_format_string
+      in: query
+      description: |
+        The URL used for the link. If you used named groups for the `pattern`,
+        you can insert their content here with
+        `%(name_of_the_capturing_group)s`.
+      schema:
+        type: string
+      example: https://github.com/zulip/zulip/issues/%(id)s
+      required: true