webhooks: Support filtering GitHub activity from private repositories.

Currently, the GitHub webhook sends activity from both public and private
repositories, which could lead to unintended disclosure of sensitive
information from private repositories.

This commit introduces a ignore_private_repositories parameter to the
webhook URL. When set to true, the webhook ignore processing activity from
private repositories, ensuring that such activities are not posted to
Zulip streams. By default, if the parameter is omitted or set to false,
activities from both public and private repositories are processed
normally. This provides users with the flexibility to control the
visibility of private repository activities without altering the default
behavior.

More importantly, this introduces a cleaner mechanism for individual
incoming webhooks to declare support for settings not common to all
webhook integrations.

Fixes #31638.

zerver/openapi/zulip.yaml

@@ -14714,8 +14714,8 @@ paths:
                                 such filtering.
 
                                 **Changes**: New in Zulip 8.0 (feature level 207).
-                            config:
-                              $ref: "#/components/schemas/BotConfiguration"
+                            config_options:
+                              $ref: "#/components/schemas/WebhookConfigOption"
                       recent_private_conversations:
                         description: |
                           Present if `recent_private_conversations` is present in `fetch_event_types`.
@@ -21990,6 +21990,36 @@ components:
         description: |
           `{config_key}`: Description/value of the configuration data key.
         type: string
+    WebhookConfigOption:
+      type: array
+      description: |
+        An array of configuration options where each option is an
+        object containing a unique identifier, a human-readable name,
+        and a validation function name hinting how to verify the
+        correct input format.
+
+        This is an unstable API expected to be used only by the Zulip web
+        apps. Please discuss in chat.zulip.org before using it.
+
+        **Changes**: New in Zulip 10.0 (feature level 318).
+      items:
+        type: object
+        additionalProperties: false
+        properties:
+          key:
+            type: string
+            description: |
+              A key for the configuration option to use in generated URLs.
+          label:
+            type: string
+            description: |
+              A human-readable label of the configuration option.
+          validator:
+            type: string
+            description: |
+              The name of the validator function for the configuration
+              option. Currently generated values are `check_bool` and
+              `check_str`.
     CustomProfileField:
       type: object
       additionalProperties: false