Add new AudienceGroupType POP_AD_IMP to Audience Group API (#842)

line/line-openapi#113

# Add `POP_AD_IMP` to **AudienceGroupType** Enum

We have supported for the new audience‑group type **`POP_AD_IMP`**
(POP ad impression audience) to the OpenAPI schema.

## Changes Made

* **Updated `AudienceGroupType` enumeration**

  * **New value**: `POP_AD_IMP`
* **Description**: Audience groups generated from impressions of *LINE
Beacon Network* (POP) ads.
  * **Region**: **Taiwan‑only** at launch

## Purpose

This update enables correct identification and handling of audience
groups built from POP ad impressions, a feature that will be released
for the Taiwan market.

## Documents and Reference

- [Managing
Audience](https://developers.line.biz/en/reference/messaging-api/#manage-audience-group)

For more information, please refer to the links provided above.

Co-authored-by: github-actions <github-actions@github.com>

Author: github-actions[bot]

line-openapi

@@ -41,6 +41,7 @@ class AudienceGroupType(str, Enum):
     IMAGE_CLICK = 'IMAGE_CLICK'
     RICHMENU_IMP = 'RICHMENU_IMP'
     RICHMENU_CLICK = 'RICHMENU_CLICK'
+    POP_AD_IMP = 'POP_AD_IMP'
 
     @classmethod
     def from_json(cls, json_str: str) -> AudienceGroupType: