Receive CloudEvent and URL query parameters

CloudEvent signature type function is really useful.

I'm working on build a flow as below.
```
Pub/Sub topic A -> Eventarc A -v
Pub/Sub topic B -> Eventarc B ---> Function
Pub/Sub topic C -> Eventarc C -^
```
I would like to receive CloudEvents from several topics in the same Function instance and adjust the processing somewhat for each topic.

Eventarc allows to specify a service URL path when invoking a destination Function. So, I adjust the behavior of Function to fit the Pub/Sub Topic by using URL query parameters like `/?mailTo=foo@example.com`, `/?mailTo=bar@example.com` and so on.
This service URL path can be easily edited from the Cloud Console for ease of operation.

The current CloudEvent signature Function does not seem to have a way to obtain URL query parameters.
So I use the http signature type Function and unmarshall the Pub/Sub message and control the http response myself.

If we can use URL query parameters in CloudEvent signature type, we can build a more convenient and flexible system.
Is there a way to recieve URL query parameters in CloudEvent signature Function?
Or is there another better way?

Thanks.

この issue にはファイルもテストも記載されていません。まず functions-framework-nodejs における CloudEvent 署名リクエストの処理を追跡し、URL クエリパラメータが公開されているかどうかを確認してください。アクセスするための文書化およびテスト済みの方法、または maintainer が確認した代替手段が得られれば完了です。

索引モデルが issue の本文から書いたものです。