Add new AudienceGroupType POP_AD_IMP to Audience Group API (#612)

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 <[email protected]>

Author: github-actions[bot]

line-openapi

@@ -51,4 +51,6 @@ const (
 	AudienceGroupType_RICHMENU_IMP AudienceGroupType = "RICHMENU_IMP"
 
 	AudienceGroupType_RICHMENU_CLICK AudienceGroupType = "RICHMENU_CLICK"
+
+	AudienceGroupType_POP_AD_IMP AudienceGroupType = "POP_AD_IMP"
 )