Add audience targeting fields to Content Import Source Preview spec

Add audience_id and apply_audience_to_existing_content fields to the
Preview OpenAPI spec (descriptions/0/) for Content Import Sources, per
intercom/intercom#497303.

Changes:
- content_import_source response schema: add audience_id (nullable integer)
- create_content_import_source_request: add optional audience_id
- update_content_import_source_request: add optional audience_id and
  apply_audience_to_existing_content (boolean, default false)
- All inline response examples updated with audience_id: null

Author: ib-skoric

descriptions/0/api.intercom.io.yaml

@@ -642,6 +642,7 @@ paths:
                       sync_behavior: automatic
                       created_at: 1734537259
                       updated_at: 1734537259
+                      audience_id: null
                     - id: 34
                       type: content_import_source
                       last_synced_at: 1734537259
@@ -650,6 +651,7 @@ paths:
                       sync_behavior: automatic
                       created_at: 1734537259
                       updated_at: 1734537259
+                      audience_id: null
                     - id: 35
                       type: content_import_source
                       last_synced_at: 1734537259
@@ -658,6 +660,7 @@ paths:
                       sync_behavior: automatic
                       created_at: 1734537259
                       updated_at: 1734537259
+                      audience_id: null
                     pages:
                       type: pages
                       page: 1
@@ -709,6 +712,7 @@ paths:
                     sync_behavior: api
                     created_at: 1734537261
                     updated_at: 1734537261
+                    audience_id: null
               schema:
                 "$ref": "#/components/schemas/content_import_source"
         '401':
@@ -801,6 +805,7 @@ paths:
                     sync_behavior: api
                     created_at: 1734537265
                     updated_at: 1734537265
+                    audience_id: null
               schema:
                 "$ref": "#/components/schemas/content_import_source"
         '401':
@@ -844,6 +849,7 @@ paths:
                     sync_behavior: api
                     created_at: 1734537267
                     updated_at: 1734537267
+                    audience_id: null
               schema:
                 "$ref": "#/components/schemas/content_import_source"
         '401':
@@ -20957,6 +20963,11 @@ components:
           format: date-time
           description: The time when the content import source was last updated.
           example: 1672928610
+        audience_id:
+          type: integer
+          nullable: true
+          description: The unique identifier for the audience associated with this content import source.
+          example: 5678
       required:
       - id
       - type
@@ -22287,6 +22298,11 @@ components:
           type: string
           description: The URL of the content import source.
           example: https://help.example.com
+        audience_id:
+          type: integer
+          nullable: true
+          description: The unique identifier for the audience to associate with this content import source.
+          example: 5678
       required:
       - sync_behavior
       - url
@@ -28395,6 +28411,16 @@ components:
           description: The URL of the content import source. This may only be different
             from the existing value if the sync behavior is API.
           example: https://help.example.com
+        audience_id:
+          type: integer
+          nullable: true
+          description: The unique identifier for the audience to associate with this content import source. Set to null to remove the audience.
+          example: 5678
+        apply_audience_to_existing_content:
+          type: boolean
+          description: When true, the audience will be applied to all existing external pages belonging to this content import source.
+          default: false
+          example: false
       required:
       - sync_behavior
       - url