Add support for Pub/Sub

## Current Behavior

Pub/Sub messages are incompatible with protojson parsing. This is because Pub/Sub messages include duplicate fields `message_id`/`messageId`, and `publish_time`/`publishTime`, which the protobuf spec indicates is invalid.

Note: parsing can also error on the "@type" field

## Expected Behavior

Tooling that allows for parsing of duplicated fields.

首先定位 Java protojson 解析路径和 Pub/Sub CloudEvent 消息定义；issue 没有指定任何文件或测试。使用重复的 message_id/messageId 和 publish_time/publishTime 字段重现解析过程，然后检查 @type 情况。当工具能够接受这些 Pub/Sub 消息且不出现解析错误时，即表示完成。

由索引模型根据 Issue 内容生成。