Add PubSub support

[lucy-yuan]

Create extension methods to convert between CloudEvent and PubsubMessage

[jskeet]

Just to check, do you mean as per https://github.com/googleapis/google-cloudevents/blob/main/docs/spec/pubsub.md?

[lucy-yuan]

Yes, exactly

[jskeet]

Okay. As a maintainer of both the Google.Cloud.PubSub.V1 library (which we'd presumably want to depend on) as well as the CloudEvents SDK, I'm in a good position to look at this - but probably not imminently.