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.

Start by locating the Java protojson parsing path and the Pub/Sub CloudEvent message definitions; the issue names no files or tests. Reproduce parsing with duplicate message_id/messageId and publish_time/publishTime fields, then check the @type case. Done means the tooling accepts these Pub/Sub messages without parsing errors.

索引モデルが issue の本文から書いたものです。